Probably a cache-ing issue, where Roku or something somewhere 'remembers' the last image. If you search this section for 'cache' you'll likely find some posts with similar issues.
For starters, I would try renaming the images/thumbnails to things like "Thumbnail2" "MoviePosterThumb2" etc so that the image has a new name (ie. Not just keeping the previous name but putting a new image up on the server host). Secondly make sure to do the same to the json file, don't just change the feed, change the title to so it's
www.domain.com/json2 or something. Again, cache.
Things like that. Also I think somewhere in the Roku settings (System > Settings or Settings > System etc) is an Update Channels option, kinda forces your device to re-grab all channel feeds & images etc. I think that's worked for people in the past. Granted you don't want all your customers to have to do that, but it'll at least help narrow down what & if you can do at this point. Again I would try the above renaming as well.