Just thought I would expand upon a comment made for this MOD.

Adding as many brushes as you like is easy:

To add to BP's comment to Ello:

Simply add a transparent png to C:\Users\*YOURNAME\AppData\Local\Colossal Order\Cities_Skylines\Addons\MapEditor\Brushes

I created (using paint[dot]net 4.0.8) a 32bit 25x25 transparent png added to the directory above and hey presto. If it is not transparent your brush will be a square brush of 25x25.

Or find a tranparent png you like.

Be aware the more brushes you add the longer it takes to load each time you click the terrain button in either the map editor or BP's Mod

Enjoy

PS: See this for further details

http://community.simtropolis.com/forums/topic/67071-326-brushes-more-for-the-map-editor/