event.target.buttonImportIcon();

Someone requested more info, so here's a more complete script that can be placed in a folder-level JavaScript file:

var ASGJ_buttonImportIconAll = app.trustedFunction(function (doc, f_name) {
    // Get a reference to the specified field
    var f = doc.getField(f_name);
    // If the specified field does not exist...
    if (!f) {
        app.beginPriv();
        app.alert({
            cMsg: "The specified field [" + f_name + "] does not exist.",
            nIcon: 0,  // Error icon
            nType: 0,  // OK
            cTitle: "Field does not exist"
        });
        app.endPriv();
        return;
    }
    // If the specified field is not a button...
    if (f.type !== "button") {
        app.beginPriv();
        app.alert({
            cMsg: "The specified field [" + f_name + "] is not a button.\r\rField type: " + f.type + "\r\rCannot import icon.",
            nIcon: 0,  // Error icon
            nType: 0,  // OK
            cTitle: "Field is not a button"
        });
        app.endPriv();
        return;
    }
    // If the button layout is label only...
    if (f.buttonPosition === position.textOnly) {
        app.beginPriv();
        app.alert({
            cMsg: "The specified button [" + f_name + "] has a Text-only layout.\r\rCannot import icon.",
            nIcon: 0,  // Error icon
            nType: 0,  // OK
            cTitle: "Button layout is text-only"
        });
        app.endPriv();
        return;
    }
    // Prompt user to select a file to use as the source for the button icon
    app.beginPriv();
    var oFilespec = app.browseForDoc({
        cFileFilter: 2,
        cWindowTitle: "Select an image"
    });
    app.endPriv();
    // Deal with the selected file
    if (typeof oFilespec !== "undefined") {
        // Attempt to use the selected file as the source of the button icon
        app.beginPriv();
        var nReturn = f.buttonImportIcon({
            cPath: oFilespec.cPath
        });
        app.endPriv();
        switch (nReturn) {
        case -1 :
            app.beginPriv();
            app.alert({
                cMsg: "The selected file: " + oFilespec.cPath + "\r\rcould not be opened." ,
                nIcon: 0,  // Error icon
                nType: 0,  // OK
                cTitle: "Cannot open selected file"
            });
            app.endPriv();
            break;
        case -2 :
            app.beginPriv();
            app.alert({
                cMsg: "The selected page is invalid.\r\rCannot import icon.",
                nIcon: 0,  // Error icon
                nType: 0,  // OK
                cTitle: "Selected page invalid"
            });
            app.endPriv();
            break;
        default :
            // Import was successful or user canceled
            // so there's nothing to do
        }
    }
});

The function can be called like this in the Mouse Up event of a button:

ASGJ_buttonImportIconAll(this, event.target.name);