Seashore is open source image editing software for Macs, from Sourceforge.net:
[Seashore] features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMP’s technology and uses the same native file format.
…Seashore … aims to serve the basic image editing needs of most computer users, not to provide a replacement for professional image editing products.
Featuring a very Photoshop-like interface, Seashore provides much more than “basic” photo editing. Layers, merging effects, channel editing, transparent and gradient effects, and a large selection of brush strokes make this a powerful image editor, even for more advanced users.
Seashore’s features include:
- Full support for the GIMP’s native XCF file format
- Read and write support for the TIFF, PNG, JPEG, JP2000 and GIF file formats
- Read-only support for the BMP, PICT, PDF and XBM file formats
- Layers with over 20 merging effects
- Ability to edit just the primary or alpha channels
- Thorough transparency effects including semi-transparent gradients
- Arbitrary selection regions
- Anti-aliased brush strokes
- 6 basic gradient effects with 16 variations
- Tablet support
- ColorSync support (including embedded profiles in TIFFs and CMYK previewing)
- Plug-in filters
Most features work on Mac OS 10.3 and later, with the exception of JP2000 support for which Mac OS 10.4 is required.
Of all the open source editors I have tried (as well as several commercial programs), this one looks like a front-runner!
As with any image editing software, be sure to work with copies of important work, rather than the originals.
Article update July 8, 2014:
Seashore has continued improvements and updates. The 2014 version is available here:
http://sourceforge.net/projects/seashore/
Download earlier versions of Seashore here: http://seashore.sourceforge.net/download.php
