Blurry Pictures and Image Class

I’ve been having a recent problem where my images show up slightly blurry. I’m not resizing them on WordPress or anything so I tried removing anything unnecessary from the img tags. I took out all of the img class information (class=”aligncenter size-full wp-image-1632″) and suddenly my pictures were crisp again. Any ideas as to what is going on here? And is there a way to insert images without the class information? It’s kind of a nuisance to have to constantly delete that stuff and add in separate alignment tags when I’m putting together a photo heavy post.