Yeah. I double checked just now to be sure of what I'm doing. When I put a .png file into any folder on my SD card, the file doesn't show up in my gallery and doesn't show up when I try to select a custom icon in LauncherPro. If I just change the file's extension to .jpg and put it in the same location on my SD card, the image shows up no problem.
Edit: Apparently my problem was the USB connection option I was using. I switched from PC Mode to Mass Storage Mode and now the files are showing up correctly.