For our purposes, the bigger the image size the better the slide. Website, Instagram and Facebook images are all low resolution, for optimum slide quality we require high resolution images.
Low resolution images don’t produce pixelated slides, but they do produce blurred slides. Whether this matters depends on the size of projection, the distance of the viewer from the screen, and expectations of the customer. Of course, there may be no alternative image available, so you have no choice but to put up with sub-optimal quality. That all said, many people are more than happy with the results. If you are in any doubt then why not email an image over and ask for advice?
For use on SUCK UK light boxes etc, with no magnification, low resolution images are perfectly fine.