There are several open source software options available for photo editing. Some popular options include:
GIMP: GIMP (GNU Image Manipulation Program) is a free and open source photo editing software that is available for Windows, Mac, and Linux. It is a powerful tool that offers a wide range of features, including support for layers, masks, and filters, as well as support for a variety of file formats.
Krita: Krita is a free and open source digital painting and illustration software that is available for Windows, Mac, and Linux. It is designed for artists and offers a wide range of features, including support for layers, brushes, and effects, as well as support for a variety of file formats.
Darktable: Darktable is a free and open source photo management and editing software that is available for Windows, Mac, and Linux. It is designed to be a digital negative (DNG) workflow tool and offers a wide range of features, including support for non-destructive editing, high dynamic range (HDR) merging, and support for a variety of file formats.
RawTherapee: RawTherapee is a free and open source photo editing software that is available for Windows, Mac, and Linux. It is designed for editing raw files and offers a wide range of features, including support for color management, noise reduction, and support for a variety of file formats.
digiKam: digiKam is a free and open source photo management and editing software that is available for Linux. It offers a wide range of features, including support for tags, ratings, and metadata, as well as support for a variety of file formats.
No matter which open source photo editing software you choose, be sure to read through the documentation and get familiar with the interface and features before you start editing your photos.