What are some alternatives?
When comparing Sketch and Adobe Photoshop, you can also consider the following products
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
GIMP - GIMP is a multiplatform photo manipulation tool.
Affinity Designer - Professional creative software, exclusively for Mac.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Adobe Illustrator - Adobe Illustrator is a vector graphics editor.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.