I often receive logos and photographs from outside sources that are too small/lo-res to work in a print document. The rule of thumb is that a file needs to be 300 dpi at its displayed size. A jpg or tiff will do. An eps for print is preferred. 95% of the time, anything pulled off of a website will not work. Knowing this information will not only save time with your graphic designer or printer, but overall it will make your final printed document so much better.