You can include folder location in the name argument, but the "home" for screenshots will always be the AssaultCube home folder.
For example,
will save Example.jpg in \screenshots\tests -- if that folder exists before you execute that command. I'm not sure whether using dots to go "up" a folder would work:
to save, in this example, in the folder one up from the AssaultCube "home"...
EDIT: Tested, yeah, it works that way.
For example,
[SELECT ALL] Code:
/screenshot screenshots\tests\Example.jpg
[SELECT ALL] Code:
/screenshot ..\Example.jpg
EDIT: Tested, yeah, it works that way.