Thanks for your suggestions, but that won't really solve the problem.
The picture is oversized independently from browser and his zoom.
That is IMHO an serverside issue.
It does solve the problem: image is too big, so zoom out to see the whole thing.
The server is displaying the image as uploaded, sure some sites will shrink the image to fit the browser window but not this one as of yet.
It looks like the poster may be able to set an image width:
https://www.simplemachines.org/community/index.php?topic=363726.0But any sort of resize plugin I saw from a quick search modifies the original image, and I believe is not something the mods want to deal with:
https://custom.simplemachines.org/mods/index.php?mod=2206