Gimp-issue

I want to edit the Warriors-dorna, the problem is that when I tried to do that with Gimp when I try to save the edited .dds a window appears that says it's not possible to generate mipmaps whose dimensions are not powers of 2!
The resolution/dimensions of the Warriors-dorna .dds are 2560x128...what can I do to edit the dorna?
The problem is that if I don't generate the mipmaps the old dorna is behind the new one visible!

Ah, and further another question...if I hex edit a file, let's say a court, there I hex-edit the wood!
Does the hex-edit affect the wood-texture itself?Let's say I hex edit the f001.iff and if I take the textures of f001.iff and import them with the Modtool to another floor, do I transfer with the texture also the hex-edit?

thanks in advance!

no idea about the gimp issue

but for the floor, the wood color goes along with the floor texture, they combine.