Picture files slow to open and not processing when being uploaded?

All of the picture (.jpg, .png, etc.) files on my MacBook Air have the default icon as their thumbnail even though I have turned on the setting to show a thumbnail. Also whenever I try to upload one of them online somewhere, before I can click "Open" the entire browser freezes and my mouse turns into the rainbow pinwheel of death. My document files also have no thumbnail, although I have no tried to upload any of them to anything yet. I've tried switching browsers and checking for viruses, nothing has changed. It's becoming very inconvenient.

Help? Any ideas?