AFAIU, there is already title support. It should be straightforward to update it:
https://github.com/xwiki/xwiki-platform/blob/2ed742eb5385d97d1128de8b693798b8b118ae07/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-war/src/main/webapp/resources/uicomponents/widgets/gallery/gallery.js#L66
Note that the title attribute on images should be uncommon: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/img#the_title_attribute We probably want to keep supporting it mostly for backwards compatibility.
