Well for example, if your working with a 400 dpi image, it'll be a pretty big canvas, if you import a smaller web found image, most of which are 72dpi or so, they will be really tiny on the page.
Only thing you can really do is find a stock site and grab high dpi images.
Depends solely on the images you need, tell me what your after and i should be able to give you some decent links to high dpi photos and stock.