We’ve added Show/hide films you’ve customized filters on the web and in our apps. (These will be extended to support backdrops as well.) Enable the Show version of the filter while browsing your own watched films, or All Films (from the Browse tab / main Films section), to see your full list of poster customizations.
The above-mentioned filter does not show films that have been individually customized within a single diary entry/review, list or profile (see Can I change a poster or backdrop on a particular list or review, or on my profile?). The Change Poster/Backdrop modal dialog on our desktop website has a link to each of your lists that includes an individually customized poster for the film in question.