I would say that the image has been widened or stretched. Those aren't technical terms, however. There probably are some technical terms for the distortions of images the people can achieve with programs like Photoshop or GIMP, but I am not familiar with them.
Some software will call this resizing, another word for scaling. These words don't necessarily give the idea of distortion, however, because you could be adjusting the image size/scale with a locked aspect ratio to enlarge or shrink the whole image evenly.
If the idea of distortion is important, I recommend stretched.