Creating a slideshow with images and text overlays
I don't believe that there's a module that supports the dynamic placing of text on top of an image in a wysiwyg editor. However, you should be able to use the imagecache_actions module to superimpose a text field onto an image field; IOW, you'd need to have separate fields (or a field collection) for the image and text. It also supports custom actions which should allow you to exploit the imageapi and GD as per your requirements.