The Hydra Commons Wiki is a file repository used to store widely used files for all Gamepedia wikis, and which is accessible by default from all other wikis. This includes achievements, templates, and promotional images. Only Gamepedia staff can upload files to Hydra Commons and edit the wiki, so make a request to one of them if you have a file you would like uploaded.
All files on Hydra commons can be referenced as if they existed on the target wiki. No special markup is required.