Most presentation designers suggest using full bleed images. This term comes from photo printing and means that instead of laying down ink right up to the border, the image goes all the way to the edge of the paper. Full bleed images in presentations take up the entire screen space.
Framed images, in your presentation, are set within the boundaries of your screen, rather than going right up to the edge like a full bleed image. Frames break up our sight lines. Our eyes naturally move around an image rather than fixating on one point. Frames define the area where our eyes will move and break up the natural flow.
I'm not against using frames in presentations, but it needs to be done with a purpose.
How do you like to show images? What rules do you occasionally break?