Ms Paint Tricks
There are many hidden features in MS Paint that most people don't know about.
Here's a video by "danjoe" covering most of them. More on tricks and in more detail can be found below.
You can scale the brush size beyond the default sizes by holding control and + (Use the numpad). You can decrease size using Ctrl and -
You can change sizes for the airbrush, line and eraser tool.
You can paint using custom brushes by selecting your brush, setting the background as transparent, then moving the brush while holding shift down(Hold down shift before you grab your brush). I drew the image above using this technique.
You can hold down ctrl before dragging your selection to create a duplicate of your selection.
3. Image Scaling.
You can scale Image by selecting your image with the Select tool, and the pressing ctrl and + (together). Shrinking an image this way keeps the proportions.
4. Color Replacement
If you made an image with a certain color and can't easily change it with the paint bucket tool, there is a quite simple solution.
Let's say you're not satisfied with the black border on this image.
You want to change it to white. Select the eraser tool, change the first color to the current color you want to change(black in this example) and set the second color as your desired color (Right click on a color, white in this example). Now just erase the color you want to change with the Right Mouse Button.
There is also a grid in MS Paint which is available through the use of hotkeys. While zoomed in at x6 or x8 press ctrl+G.