I sell pictures for people but sometimes I have to edit those pictures, remove backgrounds, make adjustments etc. and make sure they are good enough to sell. The resulting files are then sold to customers - who owns those actual files?

One client said they now want all the files so they can sell them elsewhere, but their pictures are only in a saleable condition/format because of my work editing.

I don't want this to get awkward (I know I should have something in writing) but I want to know if I'm within my rights to say no, that the new person must edit them, or if I should start to charge for editing (or apply a charge now, to take them elsewhere).

No written contract as such, just verbal and email conversations. When a picture sells we split the money.