We always make it clear in the initial negotiation that we want to own all source images, assets, and files, as well as all copyright and rights of use.

It isn't so common in adult, but in commercial graphic design, some designers still cling to the outdated notion that they retain rights to source files and copyright to the design, so we always clarify that up front so there are no misunderstandings. If the designer balks, we just go elsewhere.