I'm having a problem I hope someone can help me with...

I've been creating photoshop images, saving them as JPEGs then importing them as backgrounds for my renders. Then I set the render output to the same size as the JPEG resolution and run the render. It worked great for a while, then the background image would start to deform (x axis) in later renders. Is there a setting that I've changed inadvertantly?

Also, my scenes are crystal clear, but any background I import shows up as blurry or pixelated in the final render. I'm trying to render images at 1920x1200. 

Has anybody seen this before, and can you tell me what to do?

Thanks,
Seaview123