Rotate it. Not flip.
or you could just follow what I suggested. btw, its only useful for some images. Not applicable for .tga files. Its alright for jpg, png, gif etc.

You mean change the direction of the texture (left to right) ? Crop half of the texture , move the upper side of the texture to the lower side and put the lower side on the upper side , or simply ...
Crop to half then switch the two part ...
Hope this Help

Why do you need to crop? If you crop it, you cant read it anymore. It is simply flipping and rotating the image.