This is because BBs are drawn directly to the frame buffer, whereas Pictures are texture mapped planes, which rely on the underlying mapping mechanism of the GL… and if the images being used are large, that will only add to the “blurry” effect. You can try turning off the “Filter” option in the material (if it isn’t off already), but that’s pretty much all I can suggest or do about it…
Note: Using larger resolution images will not improve anything, especially if the resolution exceeds the limit of your GPUs texture size. This is not a resolution problem, it’s a mapping and texel resolution problem.
That being said…if you have examples where this is really bad, I’d like to see them.