If it works in Brave but not in Chrome, I suspect that this is because you snapped the image while in Brave, but the image in Chrome is slightly different (more likely a different size.) Each browser has its own zoom level, and perhaps Brave and Chrome have set their zoom levels to different values.
Have you tried rebooting the KM Engine lately? Depending on your amount of RAM, Find Image can fail to work after a long uptime. I usually reboot weekly on my 24 GB Mac in order to prevent problems with Find Image. On my old 8 GB Mac, I had to reboot daily, sometimes hourly.
Is Chrome the active front window when you are doing your image search? According to some posts, Chrome has a bug that prevents that from working correctly:
As for "main window" I don't understand what that term means. In any case, your questions about windows are probably pertaining to your Find Image issue, right? Are you asking about one of the pull-down menu items in the Find Image action? There is no "main window" option but there is a "main screen" option.
I use only Safari, so and I've never had a problem with using KM's Find Image action on it. I also use Safari because I trust its privacy, especially when using "Private Relay," which can do things that even VPNs cannot do. I certainly don't trust Chrome for anything. Plus I also adore Safari's Reader Mode, which I don't think exists on other browsers.
The same one in which you defined the image to find...
Different browsers can render the same page differently, and that includes images on the page. But any browser (at least within versions!) should render a page consistently -- although there's always window size, page code induced changes, etc that can change an image -- and it's that consistency you want for your action.