To do this you either need some painting skills or already have a graphic you can edit. Your spray logo has to be a bitmap (.bmp) with 32 pixels × 32 pixels and must be grayscaled. Put the file into the subfolder "logos" of CS2D.
Remember: A white pixel is fully opaque and will be tinted into the color you select in the options menu while a black pixel is completely invisible. A gray pixel is semi-transparent; the more black, the more transparent.
To use your spray go into the "Options" menu and select the "Player" tab. Click on the spraylogo until your logo appears. Select a color afterwards.