Another guess here, but it could have been crowdsourced using Amazon Mechanical Turk. Assuming a conservative 0.05$ by picture, the total cost would be 901 * 0.05 = 45.05$.
You can tell he/she "cheated" a bit for some the parts of the canvas that didn't get many seeds. For example, if you move your mouse within one of the large cells in the bottom left, the image just moves to keep up with the pointer. :P