Place many PNGs on many different backgrounds

Here is the final 1.4 script, there is no longer any need to use an action or batch/image processor/image processor pro as this script is now self-contained. You will be prompted to select the JPG, PNG and Save folders and the script will then create the final combined JPG files with random backgrounds.

/*

Place Random Background Image.jsx
Version 1.4, Stephen Marsh, 3rd April 2022 
https://community.adobe.com/t5/photoshop-ecosystem-discussions/place-many-pngs-on-many-different-backgrounds/td-p/12839669

Notes:
  * Random background images in JPEG format, 1000 x 1000 px
  * Random background image filename must be consistent and have variable digits, 
    i.e.: bk (5).jpg, bk (20).jpg or Post_4.jpg, Post_11.jpg etc
  * Variable foreground images in transparent PNG format, various sizes fitted to 800 x 800px
  * Foreground to be centered on background

*/

#target photoshop

    (function () {

        try {

            // JPEG folder selection
            var jpgFolder = Folder.selectDialog("Select the random JPG background image folder:");
            if (jpgFolder === null) {
                alert('Script cancelled!');
                return;
            }

            // PNG folder selection
            var pngFolder = Folder.selectDialog("Select the transparent PNG foreground image folder:");
            if (pngFolder === null) {
                alert('Script cancelled!');
                return;
            }

            // Validate input folders
            var validateInputDir = (jpgFolder.fsName === pngFolder.fsName);
            if (validateInputDir === true) {
                alert("Script cancelled as both the input folders are the same!");
                return;
            }

            // Limit the file input
            var jpgList = jpgFolder.getFiles(/\.(jpg|jpeg)$/i);
            var pngList = pngFolder.getFiles(/\.(png)$/i);

            // Validate that the JPEG list isn't empty 
            var validateJPEGEmptyList = (jpgList.length > 0);
            if (validateJPEGEmptyList === false) {
                alert("Script cancelled as the JPEG input folder is empty!");
                return;
            }

            // Validate that the PNG list isn't empty 
            var validatePNGEmptyList = (jpgList.length > 0);
            if (validatePNGEmptyList === false) {
                alert("Script cancelled as the PNG input folder is empty!");
                return;
            }

            // Output folder selection
            var saveFolder = Folder.selectDialog("Please select the folder to save to...");
            if (saveFolder === null) {
                alert('Script cancelled!');
                return;
            }

            // Save and set the dialog display settings
            var savedDisplayDialogs = app.displayDialogs;
            app.displayDialogs = DialogModes.NO;

            // Perform the stacking and saving
            for (var i = 0; i < pngList.length; i++) {
                var doc = open(pngList[i]);
                var docName = activeDocument.name.replace(/\.[^\.]+$/, '');
                activeDocument.resizeCanvas(UnitValue(1000, "px"), UnitValue(1000, "px"), AnchorPosition.MIDDLECENTER);
                // Count the JPEG files
                var bgCount = jpgList.length;
                // Random generator based on the JPEG files count
                var bgNumber = Math.floor(Math.random() * bgCount) + 1;
                // Random background file name using random variable
                var randomBaseName = jpgList[0].displayName.replace(/\d+/, bgNumber);
                var bgFileName = jpgFolder + "/" + randomBaseName;

                placeFile(bgFileName, 100);
                centreActiveLayerToCanvas();
                moveLayerRelativeStack("previous");

                // JPEG save options
                var jpgOptions = new JPEGSaveOptions();
                jpgOptions.formatOptions = FormatOptions.STANDARDBASELINE;
                jpgOptions.embedColorProfile = true;
                jpgOptions.matte = MatteType.NONE;
                jpgOptions.quality = 10;
                doc.saveAs(new File(saveFolder + '/' + docName + '.jpg'), jpgOptions);
                doc.close(SaveOptions.DONOTSAVECHANGES);
            }

            // End of script
            app.displayDialogs = savedDisplayDialogs;
            app.beep();
            alert('Script completed!' + '\r' + pngList.length + ' JPEG files (quality level 10) saved to:' + '\r' + saveFolder.fsName);

        } catch (err) {
            while (documents.length > 0) {
                activeDocument.close(SaveOptions.DONOTSAVECHANGES);
            }
            alert("An unexpected error has occurred!");
        }


        /***** FUNCTIONS *****/

        function centreActiveLayerToCanvas() {
            var doc = app.activeDocument;
            var docLay = app.activeDocument.activeLayer;
            docLay.translate(doc.width / 2 - (docLay.bounds[0] + docLay.bounds[2]) / 2,
                doc.height / 2 - (docLay.bounds[1] + docLay.bounds[3]) / 2);
        }

        function moveLayerRelativeStack(relPos) {
            // "previous" or "next" or "front" or "back"
            var c2t = function (s) {
                return app.charIDToTypeID(s);
            };
            var s2t = function (s) {
                return app.stringIDToTypeID(s);
            };
            var descriptor = new ActionDescriptor();
            var reference = new ActionReference();
            var reference2 = new ActionReference();
            reference.putEnumerated(s2t("layer"), s2t("ordinal"), s2t("targetEnum"));
            descriptor.putReference(c2t("null"), reference);
            reference2.putEnumerated(s2t("layer"), s2t("ordinal"), s2t(relPos));
            descriptor.putReference(s2t("to"), reference2);
            executeAction(s2t("move"), descriptor, DialogModes.NO);
        }

        function placeFile(file, scale) {
            try {
                var idPlc = charIDToTypeID("Plc ");
                var desc2 = new ActionDescriptor();
                var idnull = charIDToTypeID("null");
                desc2.putPath(idnull, new File(file));
                var idFTcs = charIDToTypeID("FTcs");
                var idQCSt = charIDToTypeID("QCSt");
                var idQcsa = charIDToTypeID("Qcsa");
                desc2.putEnumerated(idFTcs, idQCSt, idQcsa);
                var idOfst = charIDToTypeID("Ofst");
                var desc3 = new ActionDescriptor();
                var idHrzn = charIDToTypeID("Hrzn");
                var idPxl = charIDToTypeID("#Pxl");
                desc3.putUnitDouble(idHrzn, idPxl, 0.000000);
                var idVrtc = charIDToTypeID("Vrtc");
                var idPxl = charIDToTypeID("#Pxl");
                desc3.putUnitDouble(idVrtc, idPxl, 0.000000);
                var idOfst = charIDToTypeID("Ofst");
                desc2.putObject(idOfst, idOfst, desc3);
                var idWdth = charIDToTypeID("Wdth");
                var idPrc = charIDToTypeID("#Prc");
                desc2.putUnitDouble(idWdth, idPrc, scale);
                var idHght = charIDToTypeID("Hght");
                var idPrc = charIDToTypeID("#Prc");
                desc2.putUnitDouble(idHght, idPrc, scale);
                var idAntA = charIDToTypeID("AntA");
                desc2.putBoolean(idAntA, true);
                executeAction(idPlc, desc2, DialogModes.NO);
            } catch (e) {}
        }

    }());