Blurry

I haven't used "blogger" for very long. Any ideas as to why a few of the pictures look all choppy/lossy when you click on it? Do I need to make sure it is a certain size so it doesn't blow it up for me? I was just wondering if there is a set size for the picture, and if it is too small, will it enlarge it for you. That is the only reason I can think of as to why the pictures are so lossy.