actually, I've been working on one of these the past few days. So far, I have a rudimentary file selector with help from the src code from unity. You can scroll through all picture files in the selected directory, and zoom in and out as well as rotate the pictures if you want to read in a different orientation. For the past 2 days, I've been stuck on trying to use the zlib library so that you can just open zip files without having to decompress them in the filesystem. I know DrMDx does this, but I can't seem to find the sourcecode for it, so I've been looking on the net at how other people use zlib.
Right now, the program isn't perfect. I think the filesystem on the card can get corrupted sometimes, and directories are listed as files. My solution for this was just deleting the directory, and copying it back again from my computer. Also, there seems to be an issue rotating .bmp files. I can rotate jpgs fine, but bmp's crash my program.
If people are interested, send me a PM, and I can send you what I have so far.