The first part of doing a street painting, is deciding on what you will be painting. Not all art will reproduce well. Choosing wisely can make the difference between a messy, unrecognizable work, or a striking masterpiece.
This picture is a scan from a book of the original painting by Thomas Lawrence, entitled "The Calmady Children". We found it in a children's art book. The main reasons we chose this work were the very engaging faces of the two girls, the beautiful tones on the skin and hair, and the dramatic use of light and dark. We knew this would work well when done large on the ground with soft pastels.
Other styles that work well are masterworks from the medieval times, impressionistic paintings, and most realistic works. Of course you can always use an original piece of art for your inspiration. Using a popular painting that is well known is always safe, and will be a crowd pleaser. Remember your audience, and that this is primarily a piece of performance art.