I found the error: i don't know, but after deleting the images, didn't change the value of $image_repo_size. By that when the user tries to upload an image, instead of have space enough int he directory, he didn't can. ($quote is 10 and the user $image_repo_size war 3246)

I deleted the value of $image_repo_size of the user, and the upload is working.

Maybe the problem was i activated the plugin after the users have uploaded imagen.

Odd. We haven't seen any reports of problems with the quota.

I have the same problem - images have been deleted but quota not reset.  Where (in database) did you find $image_repo_size?  I can't find it