Im using a creature mod and the creatures it adds are too bright that they dont fit the Morrowind landscape well, how would I make their textures darker to make them feel less out of place? Is there a simple editing software I can use?

http://www.getpaint.net/index.html can open and save .dds natively. Making a texture darker can be easily done with "Adjustments-->Brightness/Contrast". When saving the texture, use DXT1 for opaque textures and DXT5 for textures with transparency. Make sure to check "Generate Mip Maps". I normally use "Super Sampling" for mipmap generation (which I find to produce much better mipmaps than at least the default gimp algorithm), but I've read that "Fant" should be at least as good.

Thank you, Pherim, for useful information about .dds file saving.

I am new mod maker from Nexus, who made Vanilla Based Notes Retexture, we already talked there.

You are welcome! Yes, I remember, of course. I just saw the new version, it's great!

One thing I forgot to mention in the post above: You can also save textures with transparency in DXT1 format, but only with 1-bit-Alpha, meaning every pixel is either fully opaque or fully transparent. DXT5 has interpolated alpha, meaning it can have gradual alpha. So for your parchment textures you could also use DXT1 because of their sharp edges. This would reduce their size as well.