Try the "Perspective Tool" (Shift-P) in GIMP. After you need to flatten the image again (Image Menu => Flatten Image).

You can use it to stretch photographed surfaces into flat squares. It can correct a lot but there are limits.

As a general hint to photo-textures: Best days for taking shots are bright and cloudy so that there is more indirect light to capture the diffuse color.

:)

Hey...This looks like the door from the game "School Of Chaos"...