Upload image full size not working

Hi, I’ve noticed that when I try to add a full size image to my post, it sometimes inserts a random twenty digit string that keeps it from actually showing up on my blog. For example:

img class=”alignleft size-full wp-image-8043 pxxgiwkdrafnjsnhipwn” src=”http://www.onelowerlight.com/writing/wp-content/uploads/2013/01/twitter.jpg” alt=”twitter” width=”225″ height=”225″

It appears that this string is the reason why I can’t see the image on my blog posts. When I try to delete it, it always comes back. However, it only comes up on some images that I upload, not all of them, and only if I select “full size.” It does appear to be retroactively affecting posts that I published before updating to the latest version, though.

What is going on? Has somebody hacked my site, or is this just a bug? I’m using the latest version of wordpress on my own server, with the picolight theme.