Why can I not apply a certain texture compression to an image?

I have two JPG images set to “Automatic Compression”. One image sets itself to “RGB 24 bit”, the other sets itself to “RGB Compressed DXT1”. That’s strange, because I’d like them to both be RGB Compressed DXT1, so I went into the one image and manually set it. However, even after I apply that change, it stays as RGB 24 bit. Why? What would prevent an image from accepting a certain kind of compression (besides transparency, which neither image has)?

OK, found the answer.

You can’t use RGB Compressed DXT1 compression on an image unless the height and width of the image is divisible by 8. It’s almost certainly a power of 2 issue.