generated image causes Thickbox failure

Seems like images generated by PHP - perhaps because they contain arguments in the URL - causes Thickbox to show nonsense characters instead.

Example:
Go to http://quidne.vacilando.org/bg and click e.g. on the first image in the gallery in the right column. OK, it opens. The path to it is a path to a JPG file.
Now go to the last image in the gallery - that it the only one that is generated (needed resampling because the original is too large) and click it. The screen darkens, as if the image was coming up, but then the dynamic circle disappears and nothing further happens. But scroll down... you will see a lot of nonsense characters there.
The image URL is http://quidne.vacilando.org/sites/all/modules/brilliant_gallery/image.ph... and it displays properly if used in a browser.

It has definitely not anything to do with the size of the image. I tested the same image in original (2.7 MB), took forever, but showed in Thickbox, but this resampled does not show.