Backgrounds are used by MyPaint to define how the canvas looks. 
They can be selected from the Layers->Background... menu. 

You can create your own Background in this folder. 

 - Create (e.g. using GIMP) an image of size exactly 
   64*N x 64*M (in other words, 64x64, 128x128, 128x192, 256x64, 
   320x640, ... 640x192, ... etc). 
 - Create the tile graphics. This is easiest done in GIMP, by 
   creating a new image of the correct pixel size, or by cropping 
   an existing image. Remember that it should look good  
   "tiled", i.e. placed next to itself over and over in all 
   directions. 
 - Give it a good name and save it as a PNG image *without* 
   alpha channel. Put it in this folder
 - Restart MyPaint.