Anytime you make changes, such as altering a graphic or installing a new plugin, go through your site from the user end and make sure everything looks and works fine. Some plugins can cause compatibility issues with other plugins, so things like comments, user accounts and even page layout can be affected, costing you readers and making your site unusable.
I come from a family filled with members who are terribly afraid of things. My grandmother never got on a plane in her entire life because she was afraid it would crash. My mother is worried about falling down the stairs, even though she's perfectly able-bodied and fit. They both think about the bad possibilities that could, but probably won't happen. While there's certainly something to be said about being cautious, the line of letting fear cow you out of doing something is toed far too often.