No, it's your art package. DoW uses bottom-left as the origin for co-ordinates (IIRC) but TGA can optionally use top-left. When you save the image then try using bottom-left as the origin (The Gimp has an option that you tick to use top-left, with unticked being bottom-left) and it should work fine. Why the art package isn't just saving it with its original co-ordinate origin I don't know.
As for the skin, my first bit of feedback is something I give to everyone who starts with compilations/modifications.
Always make sure that you get permission before re-using other people's textures, badges, banners, etc. Some people don't mind, some people will pretty much give permission to anyone who asks, but using someone else's work means you're using their time and effort and if you're doing that then you should make sure that they don't mind you re-using their work.
As for the texture itself, it looks like you've managed to put it together quite cleanly. Personally, I'm not a fan of quite so many checks but if it fits your chapter then that's great. Some of the colours could possibly be toned down a bit, though, and the chequered bolter stands out a bit against the rest of the texture.
Thanks for sharing your first work