Your base score is based on how much of the picture you erased. You receive 20 points for each percent of the picture you erased. Thus, if you completely erased the picture, you'd get 100 * 20, or a base score of 2,000 points. If your percent complete was 92.4 percent, you'd get a base score of 92.4 * 20, or 1,848.
Finally, if you completely erased the picture, or if you used every bucket and tool, you receive a time bonus. The time bonus formula is:
Time Bonus = 3 * (Unused Time/Total Time Limit) * (Percent Complete)
Thus, if you completely erased the picture, and used exactly 4 minutes of the 5 minute time limit, your time bonus would be 3*(60/300)*(100), or 60 points. If your percent complete was 84.1%, and you used 3:30 of the 5:00 time limit, your time bonus would be 3*(90/300)*(84.1), or 76 points.
