Excellent - sorted. Thanks very much - I'd never have stumbled my way to that solution without your instructions.

It can be in the settings, as in both cases it is a simply jpeg file sent to the browser instead of a web page.
Check in Settings -> Preferences -> Advanced -> Downloads
The mime type of these images is image/jpeg
If needed, uncheck the box "hide file types opened with opera".
Select image/jpeg, click Edit to see what action is linked to it. By default it is "open with opera" here. Check also for "image/jpg" at the same time.

http://www.imcdb.org/vehicle.php?id=475616 was one of the examples of what I called your thumbs above - particularly because I wanted to see the fake castle, so it was something happening before and beyond "pics.imcdb.org", but only on your thumbs as far as I can tell and only started recently (last month or two). Could just as easily be something in the Opera settings as installed on my laptop instead of a wider issue, but I can't see an obvious way of checking/changing my settings, and am unsure/cautious about exploring technical stuff within scary-looking headings.

I installed the latest version of Opera at it seems to work fine

Does it happen only for images from "pics.imcdb.org" or also with direct-links to imageshack?
e.g. http://www.imcdb.org/vehicle.php?id=475616 where it is a direct link to the jpeg

I've used Opera as my browser for several years - generally works well but has one or two quirks. Last few weeks I've noticed that antp's thumbs won't open onscreen for me - Opera will only allow me to save them to hard drive and then open them with Picture Manager or whatever. Now, (with new picture hosting arrangements?), this is spreading to several contributors over the last few days. It only affects thumbs, not main pages. Is it an Opera quirk, or something that can be fixed centrally? Anyone else use Opera and have a similar problem?