PNG Compression level - Currently there's no control over the ZIP compression level of PNG images. I'm planning on some re-work of the PNG file format handler similar to the TIFF changes that I've been working on, that should include some control over the compression level.
GIF LZW - GIF compression is controlled completely by libungif/giflib. Unfortunately Debian/Ubuntu are still shipping libungif as the main dependency for any libraries that require the giflib ABI, and the way they have the conflicts setup is badly broken. The only real choice is to install giflib from source, but it's annoying when we use a packaging system to avoid that sort of messing around.
A common compression control - Currently there's no common compression level option you can use, though it's a good idea. It will take some thought however. The jpegquality parameter is a quality output control which happens to help file sizes when it's small, a user specifying 100% compression level is going to expect small files from PNG which exactly match the original, they might get suprised by how ugly their max compression JPEG is.
Imager as C library - Most of the imager source files are independent of perl, so you should be able to package them up into a library, the main exception being dynaload.c. I don't ship any Makefiles/configures to do that however.