[Disasterpiece]

The grey bar can be explained like this:
If the watermark gets applied and the image saved in the cache, the filesizes differs, because the watermark consumes more file size.
Now, vbulletin queries the file size from the original files and sends them in the HTTP Header so the browser knows how many bytes will be transferred. The Mod modifies this number with the cached file, so the whole image from the cache gets transferred completely. In the earlier development states i forgot this step, so grey bars appeared.
However, it's very odd that you experience grey bars, this shouldn't happen anymore.

Possible explanations are scripts and/or other mods who compress image files or use their own caching methods. Do you use any of those?

Always remember to clear your browser cache while testing the watermark behaviour. STRG+F5 should work as a clean reload.