On iPad or iPhone, I take a snapshot of the screen looking at the screen sideway, but the snapshot is taken to be in Portrait mode?
You should be able to rotate images imported from a camera in iOS' Photos, so the functionality is there. For some reason it's not enabled for synced images or screenshots.

Instead of using a computer you can rotate images with an app providing the functionality. I think Myles also means this approach. Some free apps capable of image rotation:


PhotoPad
Adobe Photoshop Express