I've been getting those too. It's a blizzard problem. There's a bandaid fix on our end.
It's not every resolution, most of mine are ok, but a few are glitched, including the fullscreen size. The quick fix is to change your resolution to take screenshots.
Working: (list may not be 100% accurate, it's jotted down haphazardly on the back of an envelope)
1600x900
1600x1024
1280x1024
1280x960
1280x768
1280x720
1152x864
1024x768
800x600
Glitched:
1680x1050
1366x768
1360x768
720x576
I would have posted all this earlier (in fact, I thought I had), but I've been struggling with gimp trying to figure out exactly how off the image is. What's happening is it starts as a big list of pixels in order. But for some reason, the code is confused about the width of the image (that's what those diagonal patterns tend to mean). For example, for the image linked above, the code makes its list of pixels of an image that's 1376 wide, but it saves the image as 1366 wide. If you adjust it back, you get something more reasonable.
![Image](https://i.imgur.com/onqAiHb.jpg)
Although the colors/detail are messed up because of jpeg. If you take your screenshots as tga, you'll get the original colors and detail. Do "/console screenshotFormat tga" once, and it should start taking your screenshots in tga. I ended up realigning the image using Irfanview instead of gimp, it takes less than a minute once you know how much the width error is.