What is happening on your site is Not a Bug. In the example you cited, someone has copied-and-pasted the preview URL from the uploader into IMG tags. This is a preview image using the wiki's defined thumbnail size.
More, the preview URL as used by the uploader uses a temporary hash because nothing has been created yet. It may not reflect the actual image once it is saved, and it might be removed by cleanup later; in that case, leaving these as broken images.
The actual image size, if the full and correct image URL is used (such as if image is inserted via the FILE tag), is actually much larger. Remove the &x=180 from any of those preview URLs and you will see the full image (even though that is still a temporary URL). Please do not use temporary URLs as if they are permanent.