As suggested I tested an image gallery based on the 'old fashioned' <table> method. It works ok in the top main eReaders. It has challenges in ReadEra (on Android) that we could not solve. For some reason that particular eReader on Android places thin borders around the text and image (cellpadding won't remove it - no CSS would remove it), and it adds a non-removable list-style too. The ReadEra eReader software code is taking precedence over inserted CSS. The software also places an unusually wide gap around the images in a table (something that cannot be overridden). We successfully wrapped the table in a DIV to center it on the page screen plus a breakpoint. This test gallery only works on Epub3 format.
Our <table> images test that works (with limitations on ReadEra via Android):
JSFiddle 1
Our DIV images test which works on a broad number of eReaders (all without hiccups so far [ADE, Pocketbook, Aldiko, Kobo, ReadEra, Moon+]) as either dual images or a single large screen image (there are 4 test examples, each a slightly different arrangement):
JSFiddle 2