Bluespray has a number of existing images on their site. One might work for you.
http://www.bluespray.net/lawns/I ended up just using Paint (basic drawing program with Windows) to make mine. It's basic, but does the job.
I first outlined the property and major landmarks from a picture I printed from Google Maps or Bing Maps. Then I scanned it in as a JPG. Re sized to 800 x 600 (required size for BlueSpray). Then used Paint to clean up and add colours. It turned out OK for my purpose,
As for loading it to your controller, it needs to be hosted somewhere on the web (I use Photobucket). To link it to your controller, click the little wrench symbol (top left corner of the lawn image). Select "Use my own image" and enter the link to the image. Click SAVE.
Here's what my image looks like (yes a bit crude)