Custom Image Header Upload… then blank page?

I’m using a theme that has the Custom Header Image upload feature. In fact, I’ve tried several themes that have this feature, and they’re all having the same issue:

When I choose the image, and click the “upload” button, it takes a few seconds, and then most of the page goes blank. The dashboard is still present, but there’s nothing else on the page — no cropping screen, no nothing. Just white space.

Any ideas? I’ve changed the wp-content/uploads CHMOD to 777. I’ve tried uninstalling and re-installing themes. I’m sorta new to all of this, and I’m out of ideas. Please help! Thanks..

CLARIFICATION: The files are, in fact, being uploaded. They appear on my server, and I can see them and access them… They are not appearing on the site, and the WordPress Dashboard seems to poop out before letting me crop. Help!! Thanks!!!