I was working with several similar images saved by other software (mostly Paint.Net or Inkscape). I could work with most of them in Photoshop with no problems. However, for a few, I could not add a new layer or perform other actions. With some experimenting I discovered that the problem was with RGBA images. Since I'm creating textures for 3D models, I need to be able to include transparency. If someone could suggest what I need to learn to work with RGBA images in Photoshop, that would be great.
RGB + Transparency has no problem adding layers in Photoshop.
But indexed color images can't have layers.
Thanks for your reply. Unfortunately, I don't know enough to understand what "indexed color images" means. However, I've found a tutorial "Photoshop Fundamentals: Working with Transparency" adapted from The Non-Designer's Photoshop Book. I'll come back after I've studied that.