A question came up in the comments to my previous post:
Any suggestions on the best way to edit DDS files on a Mac?
The answer is simple: do not edit DDS files!!!
The problem is simple: DDS is a lossy compressed format. Every time you go from an uncompressed/losslessly compressed format (like PNG, TIFF, or BMP) to a lossy format (like DDS or JPEG) you are going to lose a little bit of image quality. So if you re-open your DDS, edit it, and resave, your graphics program is going to make a round trip and reduce quality.
Therefore the work flow should be:
- Edit the original PNG or photoshop master file.
- When done, re-export to DDS.
So each time you work, you work from a lossless or uncompressed master (like PNG or a gimp/photoshop file) and then re-export to DDS.