Running out of ideas – if it’s not the browser, not google settings and not user prefs, and all your software is up to date, it absolutely should just work (tbh, I’d expect both ebay and google to just work with any Mac fresh out of the box, without even worrying about software update).
If I scroll down real quick while google images loads, I get grey boxes too for a fraction of a second while the images load, so it may just be something to do with the internet connection – are the iMac and the laptop both connecting to the router the same way? If the iMac is connecting wirelessly, have you tried connecting it to the router with an ethernet cable instead?