It's a driver issue. I get it with my ati radeon x1200 card in C2's editor with the latest driver. Hopefully you have a newer card that can use a newer driver. Power of two textures display correctly at any size in C2's editor for me. The issue doesn't occur at export at all.
As an amateur opengl programmer I'd guess it's a mip mapping issue with the textures. Maybe you could tinker with the mipmapping settings for your driver? I posted a possible coding solution before that I've used in another project, but when ashley tried it the result had no effect on my end. In the end it may be some other obscure driver bug causing it or something.