Archives/ARCHIVEID/cfg/static/style/images/
EPrints 3 Reference: Directory Structure - Metadata Fields - Repository Configuration - XML Config Files - XML Export Format - EPrints data structure - Core API - Data Objects
By default this directory does not contain anything, but it's the place to put images relating to the style - background images, icons etc.
More importantly it's a place you can override images in: eprints3/lib/static/style/images