Beginner question..

Hi all :)

I didn't know if there were tags to browse, or memories, but basically is there any simple tutorials about how to even re-size good images/caps for basic (one image) headers? For instance - I have a pre-made layout, and my header MUST be 750x245 pixels - so how can I credibly resize say, a 666x457 image? Or any image that isn't square? What do you suggest? I obviously don't want the header to be looking squished, or stretched, and would appreciate a very beginner tutorial.

Thanks in advance, and apologies for the ignorance!