Just to view a .dds-file, get DDS Viewer.
If you're just interested in modifying skins for gameplay purposes, then there are way more efficient options to do so (without having to modify the source-code or the skin at all). Will I tell how? Nah, because I'm against such unfair advantages.

Not sure about MacOS support, but here are the nvidia plug-ins;

https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop

Suicizer :( tell me please?

I am sorry sir, but couldn't you just give me a link? I have done alot of searching on the site, and can't seem to find what im looking for.

Thanks,
Ffetz

http://www.xnview.com/fr/downloadunix.html

Nope.