You fail to mention your target medium for display. If you truely want to merge a bunch of images into 1 big PNG/GIF/etc., then GD may be a (simple) way to go. While it doesn't have any fancy GUI tool for pasting images into the result, its pretty small footprint, and if you know how you want to layout things, its pretty easy to just load & paste images.

OTOH, if you're really looking to layer some images together into a web page, you may want to google for some Javascripts.