The Eraser tool is simple to use. Just click on it to select it, then click on the part of the image that you want to erase. "Erasing" changes the erased part of the image back to the background color of the image file. This is usually white, but can be changed to other colors using the color palette.
The size of the eraser can be changed by clicking on the Options portion of the Toolbar when the Eraser is active. Use a smaller eraser to go after small details, the larger eraser to wipe out larger areas of the image.
Tip: Unfortunately, the eraser does not come in alternate shapes. But, if you want to use a round "eraser" just set your paint brush to the size and use the background color (paint by using the right-button on the mouse) -- it will work just like the eraser! After a while, this might make you wonder why the Eraser is even there. It seems pretty redundant. The best I can figure is it gives you the option of having a separate tool for erasing so that you don't have to continually change the shape and size of the paintbrush when you want to use it for actual coloring.
Here I used the eraser to isolate the portrait of Andrew Jackson from a $20 bill, and get rid of everything else. When you're doing work like this, the three most important things are a steady hand, patience, and zooming in close with the Magnifying Glass tool.
Online reader, Matti Nikki, points out something we missed about
the Eraser tool. "You see, it isn't a simple eraser tool but erasing
with right button
is a color replacer tool. Choose the color you want to replace in foreground, and the new color in background, and use the eraser tool with right mouse button -- now it will only erase the one color you have chosen in foreground! Very useful for shading, coloring black lines and many other techniques."