Pull Requests and Issues are both welcome.
General priority order is:
Pull requests get priority over Issues. Some pull requests I take as written; some I modify myself; some I will request changes before accepting them. Because I've ended up supporting a lot of libraries (20 as I write this, with more on the way), I am somewhat slow to address things. Many issues have been around for a long time.
I generally do not want new file formats for stb_image because we are trying to improve its security, so increasing its attack surface is counter-productive.