What quality are those pictures that celebs sign?

OK, so my question is, what are those pictures that celebrities sign? Like, you'll see celebrities signing pictures that people hand them but those pictures are good quality like they are the type of pictures that your school gives a few weeks after picture day. Like there professional pictures, not pictures of celebrities that you print on your printer at home. I think the standard size is 8x10 or something, but what is that type of picture quality called and how can I get a picture of my fav celeb in that quality? Thanks!