Link, don’t paste pictures into ArcMap layouts
While very convenient to simply copy & paste an image from Picture Viewer, an internet browser, or whatever into your map composition, don’t do it. Use Insert > Picture instead, and deselect “save as part of document”.
Yesterday I copy-pasted 5 images into a map composition. The original photos were less than 1mb each. The resultant Arcmap .mxd ballooned from 3mb to 60mb, took many minutes to open and save, and would not print at all. Using Insert instead, the mxd grew a few kilobytes larger and opening/closing/saving performance impact was negligible.
Think of copy-paste as junk food. Fast, convenient, easily accessible, and very bad for the wasteline!