Adding items but no thumbnail

I've just installed my Omeka. I didn't change anything. But when I add an item, I don't get any thumbnail.

As you can see on my site, , it's all very empty.

What am doing wrong?

Thank you!

bmat: I'm assuming you're adding an image file to your item. Do you have ImageMagick on your server? This needs to be available for Omeka to create derivative images.