Thread:Project:Support desk/120x240 images or thumbnails not appearing

First, some relevant info:
 * MediaWiki 1.21.1
 * PHP 5.4.17 (webhost's open beta)
 * MariaDB 5.3.12
 * And using GD for images (ImageMagick isnt an option here)

Just what it says on the tin. Only 120x240 images are not appearing on pages. The images are being generated, as they're showing up in my browser's page source, but when I visit a page, the images don't appear.

Example page with this problem: http://www.wikiwrimo.org/wiki/Participant_icon

Interestingly, one such image does render, but all the others (and there are about a dozen images of this size in total) do not. No error or anything, including in my error logs. I've checked my permissions, and the images with this problem have the same permissions as the ones that are appearing.

Images of other sizes appear fine, including larger images thumbnailed to a smaller size.

Thanks in advance, all.

Edit: Forgot to mention: this problem does not appear to be specific to a file type.