I don't think there is currently an API route for getting a local cover image (stored by Koha) from the biblionumber of the record It could be useful
You can do requests like these: https://<opac>/cgi-bin/koha/opac-image.pl?thumbnail=1&biblionumber=68225 https://<opac>/cgi-bin/koha/opac-image.pl?biblionumber=68225 ...and get the image back. What would the API return other than the image itself?
*** Bug 36223 has been marked as a duplicate of this bug. ***
In my opinion there is still something missing though. There is no way to know if a record actually have a local cover image or not. If you use a generated link with the biblionumber in the hope that all your catalog will have cover image then many records will be missing a cover (unless you provide a local cover image to all your records). It would be great to export the local cover image to a marc field only if it exists. we could then check the presence of this field (unimarc 856u for example) to generate a cover image link, or not.
oh I did not know the URL https://<opac>/cgi-bin/koha/opac-image.pl?biblionumber=68225 It suits my needs
but I agree with Arthur's comment, we should have a way to check if a record has a (or several) local cover But you can not use 856u in unimarc for that, this field must be reserved for the link to the online ressource itself (see https://www.transition-bibliographique.fr/wp-content/uploads/2018/07/B856-6-2010.pdf , in french). I think marc21 is less strict.
By the way, and a bit off topic, I discover that there is a weird discrepancy between IFLA definition of UNIMARC 856 field and the use of UNIMARC in France. IFLA defintion : "This field contains the information required to locate the electronic resource described by the record and/or reference files related to the described resource such as thumbnails and content pages" => the 2d indicator can be used to specify the nature of the link In France we are not supposed to use the 2d indicator, and the 856 field is supposed to contain a link to the ressource itself