Use the image-editing program Adobe Photoshop to thicken the lines of an image. Adobe's Magic Wand tool allows you to select portions of an image that have a consistent color. This means you can capture the lines of an image "without having to trace its outline," according to the Adobe Photoshop Help pages. Adobe's Stroke function gives you the option of applying a border around your selection, so when you apply a stroke to the lines of the image, the lines get thicker.
Open the line image you want to edit in Photoshop.
Go to the "Tools" menu located on the left side of the screen. Click on the fourth icon down and hold until a menu with two options appears. Highlight the "Magic Wand Tool."
Click on some portion of the lines of the image. The portion of the line selected will appear to flicker.
Go to the Magic Wand toolbar at the top of the window and click the "Add to selection" icon. Use the tool to select the rest of the lines in the image.
Go to the "Edit" menu and select "Stroke." Enter the number of pixels by which you want to increase the line. Set the color, if not black, by clicking on the color box. If you are thickening a colored line, bring the cursor over the image to activate the color selection tool and click on the color you want to copy from the image. Set the "Location" to "Center." Leave "Blending" set at "Normal" with "Opacity" set to "100."
Click "OK" to thicken the lines of the image.