Do you have a mac or PC? On the PC, I use a somewhat primitive program that comes with the computer, Microsoft Paint. Type Paint in the Start menu search box. Open Paint. On the upper left hand side there should be a set of tools. If you don't see a set of tools click View > Show Tool Box. Among the tools you'll see a diagonal line, click on that, then below that choose the thickness of the line. Then choose a color at the top, below the menu bar. Then click inside the picture and drag the cursor to make lines. To make points, make little lines at the ends of long lines. Play around until you get the lines where you want them. CTRL Z is "undo", so you can undo mistakes. When you're done click File > Save as [name the file] if you want to save the marked-up picture in addition to the original. If you have a mac, there is a similar program but I don't remember the details. Command Z is "undo" on mac.