Photoshop Scripts for Image Layers (Javascript)

What you see happing is not what you write is happening.  Your images most likely have different Print DPI resolutions.   Photoshop want to maintain your Images pixel quality so will not ness with the pixels you have for you images.  A document can not have more than one resolution. So when you add a new Images layer the has the pixels from the other resolution image its size may look like it has changed.  too big or too small.   The image  has not been changed your have the exact same pixel in both documents. When you scale the layer  to match the scale of the image you pasted the image into the number if pixels will be changed and you will lose some image quality.

Centering the pasted layer is easy.  However, it will not resolve any scaling issue.  Select all to select the image canvas size then use align layers to selection. To the horizontal and vertical center of the selection.

In a script you can rename a layer name is read write and if you use Action manager scriptlistener code to duplicate the layer into the other document  you can name the layer that will be created with in the other document as part of the layer duplicate process like you can using Photoshop Layer>Duplicate UI. And the new layer should be centered.  If not You can move the layer and align to the canvas center using a select all and align layers to selection.

That script you posted mover layer 1 not the canvas.  The canvas top left pixels is  0X,0Y the bottom right is WidtpxX,HeightpxY The Script moves  the active layer, Layer 1 top left to canvas position 300x,300y.

You can get the Document Canvas Bounds and you can get the layer bounds and you can set as selection to a pixel and you can align the layer horizontally and vertically  relative to that pixel selected.  A Photoshop Script can position a layer many ways using different Photoshop tools.

Here is a quite simple script that will place in a watermark file into a document resize the watermark smart object layer relative to the Document height then position the resized layer to the bottom left or right of the document.

/* ==========================================================
// 2017  John J. McAssey (JJMack) 
// ======================================================= */
// This script is supplied as is. It is provided as freeware. 
// The author accepts no liability for any problems arising from its use.
/*
<javascriptresource>
<about>$$$/JavaScripts/PlaceWatermark/About=JJMack's PlaceWatermark ^r^rCopyright 2017 Mouseprints.net^r^rPhotoshop Script^rCustomize using first four var</about>
<category>JJMack's Script</category>
</javascriptresource>
*/
#target photoshop;  
app.bringToFront();  
var logoFile = "~/Desktop/JJMack.png"; // Watermark file should be large for resize down works better than up
var LogoSize = 10; // percent of document height to resize Watermark to
var LogoMargin = 1;                                         // percent of Document height the Watermark should have as a margin
var BottomLetf = false; // false = Bottom Right true Bottom Left 
placeWatermark(logoFile, LogoSize, LogoMargin);             // Place Watermark into the bottom of the document
function placeWatermark(Image,Size,Margin){  
if(!documents.length) return;  // if no document return
try{  
var doc = app.activeDocument; // set Doc object to active document
app.displayDialogs = DialogModes.NO; // Dialog off 
var strtRulerUnits = app.preferences.rulerUnits; // Save Users ruler units 
var strtTypeUnits = app.preferences.typeUnits; // Save Users Type units 
app.preferences.rulerUnits = Units.PIXELS; // work with pixels 
app.preferences.typeUnits = TypeUnits.PIXELS; // work with pixels 
var fileObj = new File(Image);                 // the passed file
if(!fileObj.exists){  // If file does not exits tell user 
alert(fileObj.name  + " does not exist!");  
return;  
}  
var layers = app.activeDocument.layers; // get layers
app.activeDocument.activeLayer = layers[0] // Target Top Layer
placeFile(fileObj); // Place in file the Watermark png file
activeDocument.activeLayer.resize(100 ,100,AnchorPosition.MIDDLECENTER); // Insure Place did not scale layer  
var SB = activeDocument.activeLayer.bounds; // get layers bounds 
var layerHeight = SB[3] - SB[1]; // get layers height  
var resizePercent = (100/layerHeight)*(Size/100*doc.height.value); // Percent to resize by 
activeDocument.activeLayer.resize(resizePercent ,resizePercent,AnchorPosition.MIDDLECENTER);  // Resize width and height by percentage 
SB = activeDocument.activeLayer.bounds; // get resized layers bounds  
activeDocument.activeLayer.translate(-SB[0].value,-SB[1].value); // Move resized layer to top left canvas corner 
var LayerWidth = (SB[2].value - SB[0].value); // get resized layers width  
var LayerHeight = (SB[3].value - SB[1].value);  // get resized layers height
marginSize = Margin/100*doc.height.value; // Margin size
// move resized watermark into the document lower right corner with some margin or lower left
if  ( BottomLetf) {activeDocument.activeLayer.translate(marginSize,( doc.height.value -marginSize - LayerHeight));}
else {activeDocument.activeLayer.translate((doc.width.value -marginSize - LayerWidth),( doc.height.value -marginSize - LayerHeight));}
}
catch(e) { alert(e + ': on line ' + e.line); } // inform user of error  
finally{  
app.preferences.rulerUnits = strtRulerUnits; // Restore user ruler units  
app.preferences.typeUnits = strtTypeUnits; // Restore user type units    
};  
}; 
function placeFile(placeFile) {  
    var desc21 = new ActionDescriptor();  
    desc21.putPath( charIDToTypeID('null'), new File(placeFile) );  
    desc21.putEnumerated( charIDToTypeID('FTcs'), charIDToTypeID('QCSt'), charIDToTypeID('Qcsa') );  
    var desc22 = new ActionDescriptor();  
    desc22.putUnitDouble( charIDToTypeID('Hrzn'), charIDToTypeID('#Pxl'), 0.000000 );  
    desc22.putUnitDouble( charIDToTypeID('Vrtc'), charIDToTypeID('#Pxl'), 0.000000 );  
    desc21.putObject( charIDToTypeID('Ofst'), charIDToTypeID('Ofst'), desc22 );  
    executeAction( charIDToTypeID('Plc '), desc21, DialogModes.NO );  
};