Images won't resize after changing theme

I’m in the process of updating my website, but when I changed the theme to the new 2010 WP theme, all my images that had originally been resized on the old website went to being displayed at full size.
Now I can’t even resize any image that I upload, or link to.

I have tried deactivating all my plugins, but to no avail.

Any ideas why this might be happening?

The old theme I was using was a modified K2 theme.

Cheers