I didn't even know this was possible, until I found out by mistake!
I kept using the "source folder" option, because I did not want to save my converted files to the same folder on my drive every time. Images exist in different albums and folders, so I was hoping there was a way to save the converted files into a relative output folder.
Guess what? It's possible! I just never knew because it was not explained!

This works on Linux, and Windows. New users might not know that XnViewMP's batch convert even supports "relative output folder" since no hints are provided.

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EDIT: Here is what it could look like, based on the conversation between myself, cday, and user0.
Checkbox selected
Checkbox deselected
XnViewMP already supports this, but you have to "trick" the "Folder" selection.

This is a better method, and it is more clear for the user.