PC images have gray pixels on bottom?

My images will show up fine on medium (and sometimes large) thumbnails. But when I try to enlarge them, either by opening them or enlarging the thumbnail, I get grayed out pixel areas anywhere from bottom fourth to entire image. This happens on some pics some of the time, and some pics all the time, and some pics never. AND, it might happen to an image one day and not the next. It's on images whether from the web or a camera or a memory stick. I have a new (3 months in use) HP Pavilion dv7-6b78us Entertainment PC with RADEON graphics, windows 7, Intel core 17. I'm a novice when it comes to images and graphics.