Sorry for my miscommunication. And most of the text boxes are already set to auto-fit, and the images are close enough in size they should be fine. Unfortunately, the only script I've been able to find only works one document at a time, which, I can't figure out the advantage of that.
This is pretty rudimentary, but something like this should work assuming that the same file is linked across the documents. First you'd be prompted to select a folder where all the indd docs live. Then you'd choose the image file that you want to replace, then you'd be prompted to choose the file you want to replace it with.
var fol = Folder.selectDialog("Choose the folder with indd files");
var inddFiles = fol.getFiles("*.indd");
var oldImage = File.openDialog("Choose the image to replace");
var oldImageName = oldImage.name.replace("%20"," ");
var repImage = File.openDialog("Choose the new image");
for (var i = 0; i < inddFiles.length; i++) {
try {
var doc = app.open(inddFiles[i]);
var link = doc.links.itemByName(oldImageName);
link.relink(repImage);
link.update();
doc.close(SaveOptions.YES);
} catch(e) {}
}