In general: poster plugins only download images in silent mode if "Download all available images" is checked.
Just to be clear, that's not working.
It actually never did [download the first poster]. The problem here was the fact that search results always contain some posters not related to a movie the plugin is looking for.
In the case of Amazon, the first search results page (I'm assuming this is where the plugin gets the thumbnails) normally places exact matches at the top of the page, and—there only being a few of these—the rest of the results are for box, sets, collections, or related movies. So
Download all available images typically gets 6 - 8 posters, only 2 - 3 of which are correct. The efficiency of
Silent Mode is outweighed by the need to delete the unwanted posters. I think it would be much better if
Download all available images were replaced with the option to download the first N posters on the results page. I might then use
Silent Mode with that set at 2 or 3, and be very confident I would get the best available poster, without too many unwanted ones.