This is a popular issue with new mac updates. I tried every possible tip for correcting it (deleting quicklook files, viewer options, etc.) and FINALLY found the solution on another ask site.
I just made an account so I can't answer other versions of this question (How to fix Finder won't show icon previews, Why can't I see thumbnails in the finder?, Mac OS X icon preview or thumbnail problem, etc.) So this is me asking the question and providing the answer for others.
Image transcript: Basically I opened up Activity Monitor, searched for a process called "com.apple.quicklook.ThumbnailsAgent," forced quit that process, and Relaunched Finder in the Force Quit menu. Worked like a charm.