Deference in DSLR lenses.?

I have a Canon XS. I know this Camera is able to take great photos and blow them up pretty big(from what I understand the MP has to do with how big you can blow images up.). My question is this; I want to start doing stock photography but my lens doesn't seem to get the images clear enough. Some of the photos I have taken are great, but most of them are no where near being sharp enough. Really the only ones that are sharp enough are macro photos. I have the kit lens(EF-S 18-55mm) Will a different lens really make that big of a change? I was looking at a 75-300mm zoom lens. Should I just suck it up and get a couple fixed focal length lenses? Thanks for any and all help!