Free program required: GIMP
Install GIMP and run it. Don't be scared by the 3 windows that open up. You can drag your image onto the "canvas" (where you'd draw) to open it, or select File -> Open and open the crosshair you'll be editing. I chose Accura Dot as my example for this.
Tip: Ctrl + MouseWheel will zoom in & out of an image (Additional: This is almost a global windows shortcut. It should work in your browser and other programs.. try it now). If it's too small to see and work with, zoom in.
Tip: That grey-checker-board pattern means "this area is transparent".
My 'tutorial pictures' were taken quickly but should show you exactly what needs to be done. Sorry I'm not pr0 tutorial maker.
TO CHANGE COLOR (also check out "Colors -> Map -> Color Exchange" for more control):
TO ADD TRANSPARENCY (for those JPEG crosshairs, lol), SELECT THE COLOR TO TURN TRANSPARENT:
TO RESIZE A CROSSHAIR:
TO START A NEW CROSSHAIR (Important to select "Fill with Transparency"):