The GNU Image Manipulation Program (GIMP) is a free photo and image editor.
If you're looking for a free alternative to Photoshop or a similar product, then GIMP is definitely for you. All the major file formats are supported including GIF, PNG, TIFF, JPEG, multi-resolution and multi-colour-depth Windows icon files. Most input devices are supported right out of the box to, such as pressure and tilt sensitive tablets. So whether you're looking for an application to enhance your photo collection or you're creating a company logo, GIMP does it all.
GIMP is available for Microsoft Windows, macOS and Linux.
You can find a list of GIMP tutorials here.