Not so fast buddy....Before you go pulling that 100px X 100px image off of Google and using it on a 10 ft. tall banner, you better listen to this Geeks and God minicast.
Talking about image resolution isn't really a blast, but it's one of those things that's extremely necessary to understand.
If I could point out one thing that divides professional designers from amateur designers, it would be the understanding and practice of good resolution usage. Too often, people are taking screen resolution images off the web and using them in print designs...thereby getting horrible looking product.
Or, on the other side (i actually forgot to mention this in the minicast), people often take print resolution images from their camera and don't down-rez them for the web, thereby making a really slow loading web graphic.
So, pay attention to your resolution...it's important!