GIMP is an open-source, photo-editing program available for operating systems like Linux. One of the many tools available is a "Blur/Sharpener" tool. If you have a picture that is slightly blurry, for example, using this tool can help give it a more defined look and can make it a bit easier to look at. This tool is available at any time from the main GIMP toolbar within the program.
Open the picture you want to sharpen in the GIMP software. With the GIMP program open on screen, select the "Open" button, then select your picture. The picture will display on screen.
Click the icon for the "Blur/Sharpen" tool in the GIMP toolbar (called the "Toolbox"). This icon is in the lower left corner of the toolbar and features a blue shape resembling a raindrop.
Select "Sharpen" to start the tool in "Sharpen" mode.
Click on the portion of your picture that you want to sharpen. Drag your mouse cursor over the top of the area of the picture. The blur affect that is present in the area will be removed as your mouse cursor passes over it.