3rd-party services commonly use ISBN for image links. This makes it easy to use UserJS to produce them from simple ISBN tags in the HTML. Currently though, one has to check e.g. Google Jackets, only to get them. This works, but also adds completely redundant loading. One solution: a simple unconditional (or conditioned by no Enhanced Content used - after the [% IF ( GoogleJackets ) %] etc. sections ) addition of SEARCH_RESULT.normalized_isbn in opac-results.tt; and corresponding .tt:s, for Intra etc.
You can add some code to OpacSearchForTitleIn to add an invisible HTML element to the page containing the {ISBN} placeholder.