Adobe Photoshop provides a pencil tool that gives you the power to draw directly over your picture, photograph or other image. The tool has a host of options so you can control the width and color. The drawing is done with the mouse or track pad and is embedded directly into the picture, rather than as a layer, so you don't have to worry about flattening the image later. Because of this, it is best to save your document as a copy, in case you later want to go back to the original.
Open your picture in Photoshop by choosing "File" and "Open" from the top menu.
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Choose the pencil tool from the tool sidebar (the icon is a diagonal pencil). If you do not see it, but you see a paintbrush icon, click and hold on that to reveal the pencil icon.
Click "Brush" to choose the size and style of the mark you want to make. You can choose the pixel width and special marks such as blots or irregular marks.
Click the color swatch in the toolbar to choose a color. A color selector window opens. Use the rainbow slider on the right side the window to choose which area of the spectrum you want. Click in the color window to choose your exact color. Click "OK" to save your choice.
Click and drag the cursor over your picture to draw, using the mouse or track pad. You can draw as long as you keep the mouse button or track pad depressed. When you release, the line ends and you can begin a new line.
Click "File" and "Save As" to save a copy of your file. Choosing "Save" rather than "Save As" overwrites your original file.