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.
When you update plugins, always do so one at a time to be sure you can pinpoint and correct any resulting problems. Likewise, when you change the layout, graphics or design of your site, check after each change to be sure things are still working smoothly. Be sure to check out every angle, as well, because sometimes things get broken that are seemingly unrelated to whatever maintenance you are doing on the admin side.
Staying on top of problems is definitely easier than fixing them later, and you can’t always rely on your readers to let you know when there is a problem. I once went two months with no comments on a site, only to discover that an errant plugin made comments not work, so take the time to check on your site frequently and avoid any long term complications.