Re-sized images automatically scale to thumbnail

I’m having issues with my images re-sizing automatically to thumbnail. When I use images, I insert the image into my posts at full size (I generally re-size them down below 800 x 600 before I upload them) and then use “Edit Image” to re-size my picture +/- 10% at a time until the image is a desirable size.

Here’s my problem: When I try to use the re-size option, even if I only re-size my image down by 10%,they automatically re-size to thumbnails.

Here’s an example: http://prettypessimist.com/?p=6531

The images on that page are 778 x 560 and 632 x 538 respectively. Each of them were originally sized down only by 30% and they fit the post fine. When I updated to WP 3.5, all images that I had previously re-sized are now thumbnails.

So how to I stop my images from automatically downsizing to thumbnails when I try to make any modifications to them?

I’m running Twenty-Twelve and have deactivated all plugins except Askimet and JetPack. So I’m certain it’s not a plugin issue.

Thanks!