Replace image in a template created on After Effects I've created an intro template on After Effects that has some text and the animation of a masked image. If I import this aep template to Premiere Pro, I can easily replace the text, however I can't find a way to replace the image too.

Is it possible?
I don't think it is.  As you say, you can replace text using Live Text Templates.
If you need to easily re-version comps from AE with different source material, I would suggest looking at Templater.