I drew the pbass. I don't recall the rules prohibiting participants from using models, so I used that picture as one. If you really want to know how it was really done in MS paint, here's the breakdown.
1. Sketch lines with the Curve tool. (The 6th tool on the right side of the Tool box)
2. Painstakingly pick and define custom colors. (This is done by double clicking on any color box in the color palette)
3. Use Airbrush (5th tool on the left side) and slowly fill in from top to bottom. (E.g., Bright green, darker green, brown, and black in the case of the driftwood in the back)
4. Obviously the Airbrush is messy, so I had to redraw the lines again using the Curve tool. (This explains the variable width of the lines in my avatar)
5. Use Brush or Eraser to remove any messiness outside the line
It took patience, perfectionism, and days to finish. It's not as simple as it sounds.