@valiowk There's definitely an issue searching for images with Bing/Google in Private windows and Guest windows (Bing only in Guest).
I've tested with both 5.2 Stable and 5.3 Snapshot. The search engine implementation has been changed a lot between these versions, but the errors seem similar - looks like it does GET requests when it should do POST, or something like that.
The image search engines in 5.3 are really new, and I'm sure stuff like this will be straightened out over time.
EDIT: I've also tested with Private windows in 5.1 and 5.0 and the same thing happens, so this issue has likely been around for a very long time.
I'd say report a bug, it's probably a known bug but but I've not heard about anyone else reporting it. Make sure to specify and test in 5.3 Snapshot - IMO reporting a bug for image search in 5.2 Stable is kinda pointless now that the SEs have been reworked for 5.3. Better to focus on getting it right for 5.3.
https://vivaldi.com/bugreport/