Multiple copies of images in media library

I'm not sure when this started - other than some time ago - but now when I add a new image to a blog post, an extra copy (sometimes 3 copies) is placed in my media library. Commonly, they will have the suffix "_m".

Worse, if I re-use one of these images, another copy is created and that *copy* is used in the blog post. I currently have 50+ versions of a logo I use. If I delete a copy, I have to edit the document manually to point it towards the original.

I mentioned this on the WordPress support forum, and someone suggested switching to Twenty Eleven and see what happened. Problem solved. Although my blog looks a bit poo as a result! Back to Atahualpa and the problem returns.

I'm on WP3.4 and Atahualpa 3.7.7, although the problem's been round for a couple of months now so has stretched back to Atahualpa 3.6.4 and WP 3.2, I reckon.