Yes the admin can choose the font.
There is 3 vars:
{$userName} : add the username of the user who uploaded the picture: eg: demo
{$displayName} : add the display name of the user who uploaded the picture:Demo User
{$userUrl} : add the link of the user who uploaded the picture eg: http://www.domain.com/user/demo
I'm going to add the "upload your own font" feature to this plugin in the next updates.
In the meantime if you cannot wait, you can replace any existing font for your own in /static/fonts folder via ftp
Just download the google font to your pc, unzip it, select the ttf or odf file, upload it to /static/fonts and rename it to an existing font. Enable dev mode and clear cache. The plugin now will be using your new font with the name of the old one.
It might also be interesting, for all-over patterns to allow a choice between dark and light for the end user.
Very light photos will not show the white transparent PNG I'm using, and a dark watermark might be better.
But that's just persnickity. *smiles*