Seashore is described as 'open source image editor for Mac OS. It 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 GIMPs technology and uses the same native file format' and is a popular app in the Photos & Graphics category. There are more than 100 alternatives to Seashore for a variety of platforms, including Windows, Mac, Online / Web-based, iPhone and iPad. The best alternative is GIMP, which is both free and Open Source. Other great apps like Seashore are Krita (Free, Open Source), Adobe Photoshop (Paid), Paint.NET (Free Personal) and Affinity Photo (Paid).
Krita is a professional Free and open source (FOSS) painting program. It is made by artists that want to see affordable art tools for everyone withith concept art, texture and matte painters, illustrations and comics.
MyPaint is a fast painting program with an emphasis on full-screen use. It lets you focus on the art instead of the program. You work on your canvas with minimum distractions, bringing up the interface only when you need it.
Pixlr® makes it easy to transform everyday images into stunning works of art. Whether you’re applying a quick fix to your photos or adding your personal touch with effects, overlays, or borders, Pixlr has everything you need to make your moments beautiful.
Pixelmator is a powerful, beautiful, and easy to use image editor. Featuring an elegant and streamlined single-window interface, it puts all the focus on the images you edit, rather than what's around them.