Adding SDL_image is probably better for long-term support anyways, because it supports loading a wide variety of image formats (so we avoid format wars later), and is already built for SDL (unlike lodepng).
Regarding adding a new dependency, basically you want nothing to break unexpectedly for anyone. As you said, you'll need to update the wiki documentation and pre-compiled dependencies, as well as the build files for various platforms (VS, Makefile, CMake, etc). You don't have to know all this yourself, but you should at least notify the people that do (eg. michal runs the Git builder) by contacting them or making a news post on the site or something (if you don't have access to news posts on the site just PM me and I'll set it up).
