Users upload images and select a template AS3 file, which would be used together with the images to compile into a SWF and be downloaded by the user.
Basically, use my own custom animations with the user's resource files in order to compile a new SWF at run-time.
I found a tool called "swftools", which can convert images into a slideshow, so I know something like this is partly feasible, I just don't know enough about AS3 to know how to load resources dynamically, without creating a FLA file manually.
EDIT: To clarify, I do not want to dynamically load an image from a URL or file at run-time. I want to embed the image into the SWF, but the image data is decided at compile-time.