GIMP is an open source graphics editor available for Windows, Mac and Linux operating systems. Many of its features are modeled after commercial graphics software such as Adobe Photoshop such as the magic wand and layers window. GIMP is designed primarily for editing and touch up purposes, and can perform simple tasks such as image resizing. DPI (dots per inch) is a measure of an image's resolution and can be adjusted through image resizing in GIMP.
Open GIMP. Load your image by selecting "File" from the top menu then "Open." Browse to your image and select "Ok."
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Select "Image" from the top menu, then "Image Scale." This will open the Scale Image dialogue. The size of your image will be listed at the top of the window in inches or pixels. Below this will be your "X Resolution" and "Y Resolution" both of which should be the same number. This is measured in "pixels/in." which is another way of saying dpi.
Enter a new value into the "X Resolution" textbox, This will automatically change the "Y Resolution" as well. The higher the dpi, the better the quality, but also the larger the file size. After about 300 dpi, an increase will hardly be discerned by the human eye.
Click on the chain icon between X Resolution and Y Resolution in order to have different resolutions horizontally and vertically. The "X Resolution" box designates the horizontal dpi value and the "Y Resolution" designates the vertical dpi value.
Click "Scale" to complete the task. Your GIMP window will automatically resize to accommodate the new resolution.
Do not think the highest DPI is the best. Images for the web, for instance, should have a smaller DPI.