There is a field under Tools => Options => File Location => Cover Art where you can select the folder. But I guess I don't understand the point of this, or the function, because if you 'add from file' and browse to another location, next time you use 'add from file' it recalls that location rather than the default file location you've designated. Maybe that's just indicating the directory for backups? So as is, once you browse to the location, next time you open 'add from file' it will remember the previous location.
The other problem I'm having with 'add from file' is that when I use it, it's full screen, and I cannot resize it, even though it has the resize indication in the lower right portion of the window. This is all with Windows 10 64-bit and MC 27. I can resize it without issue in MC 25 on Win 7.