Uploaded images automatically rotate?

I’m fairly new to WordPress and I’ve been working on uploading some content to my new 2.7 installation. Everything seems to be working fine and the flash uploader is running smoothly. But there is one odd quirk: whenever I upload a photo in portrait orientation, it seems to be automatically rotated back to landscape. I’ve tried it with the browser uploader too, and had the same effect…

Any ideas? Is there anything stupid I might be doing wrong?

Edit: Using Mac OS 10.5.6 w/ Firefox 3.0.5, if that’s relevant. Let me know if there are any other details that might be helpful.