You can make a very nice gradient color background using MS Paint. First, open Paint. Resize the white working area to the size you want, for example, 800 x 200 pixels. Pick a color for the right side of the gradient, for example green, into Color1 and Color2. Select the rectangle shape and outline and then Fill the entire rectangle with the color. Pick a second color, for example orange, into Color1 and Color2. Select the triangle shape and make a triangle in the lower left part of the image. See first image below.
Now do Ctrl-W and resize the image size in percentage without “Maintain aspect ratio” to Vertical = 1% (leave the Horizontal at 100%). Now expand by Ctrl-W, Resize, Vertical = 500%. Now shrink by Ctrl-W, Resize, Vertical = 1%.
At this point expand the image by repeating Ctrl-W, Resize, Vertical = 500% until the image is close to its original size (either 3 or 4 times). Finish with a final resize (without maintaining aspect ratio). Very cool; see result below.