MPlayer doesn't support still images currently. It either needs a lot of rewriting in order to support it, or ugly hacks (that won't make it into CVS). Still images are usually used in DVD-menus.
MPlayer G2 was designed to support still images as DVD menus is one of the most requested features in MPlayer. I have been looking in to porting the current G2 to MorphOS (it doesn't support DVD menus (yet), it only has the features required to support menus!) but I'm being torn between schoolwork and I never have quite enough time to fix the issues with compiling it on MorphOS. I'm hoping to have more time in the near future but I cannot make any promises at all. Another problem is that I have no idea who else might be working on this, someone might already be working on porting G2 or some other player with DVD menu support to MorphOS in which case I might be doing a lot of work in vain. I wish people would be a little more open and tell the world what projects they are working on ;). I almost started porting the latest snes9x three days ago when I noticed that someone had already done so here on the forums.