Because PowerPoint and Keynote don't have the functionality to zoom like Prezi does, they don't have to rely on such high-resolution imagery. So long as the images on a slide look good enough when projected, that will usually work fine for the presentation.

Once these images are placed onto a Prezi canvas, you might decide that you want to zoom into them. If the quality isn't good enough, you could end up with pixilation problems, which we will be covering in more detail in the next chapter. You can see an example of this here:

If you are worried about the quality of images once they have been inserted into your Prezi, just use the zoom feature and check for pixilation. If you lose quality and you know things just don't look good, then delete the image and use Prezi's Insert from web function...