My Images Are Different Sizes
To correct this you'll need to install the Regenerate Thumbnails plugin found here. There's a chance that your recent theme didn't call for photo sizes or different sizing in general.
Once installed and activated, navigate to Tools > Regen. Thumbnails and press the “Regenerate All Thumbnails” button and let it run until it reaches 100%.
If you have a LOT of images on your site (like I do on my personal blog), this process could take a while so don't get too impatient. Whatever you do make sure the page stays up the entire time for it to complete or you'll have to start all over again