I love the idea that "frontend code" is somehow not "additional development". Have fun with your "manifest" and "metadata".
I equally love the idea that hiring a web agency to build a wordpress template is somehow entirely different thing from hiring a web agency to build a not-wordpress template.
The frontend code which eventually renders the image gallery does not care whether fetched image list is produced by PHP code or is served by http server directly from filesystem.
> But image gallery does not need any "additional development". All it needs is frontend code to display the gallery nicely, some storage mechanism and a "manifest" of sorts with relevant metadata of the gallery
I can't tell if u missed a /s or genuinely think that isn't additional dev.
If that is a photo-sharing website, then yes, proper backend is needed. But for a portfolio website of a photographer, which gets updated once a month at best, static manifest will work wonders.