Josh if you are using GIMP, and you directly copy and Paste the image from texags (right click image in TexAgs from IE browser) then open GIMP, and hit CTRL-V to paste the image in... you should not have to resize...
If you notice on the bottom of the GIMP window, it will tell you what size the image is. If you need to resize the image go to IMAGE >> SCALE IMAGE >> then resize. Make sure the ChainLinks are connected to it keeps the same aspect...
GIMP should not resize the image for you... if you are copying the image direct from TexAgs and it's 800x600, it will show up 800x600 in GIMP...
Remember each time it needs to be resized it loses image quality and becomes more pixelated.