Photoshop Tutorials (Envato Tuts+)
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A free tutorial library from Envato covering Photoshop techniques: photo retouching, compositing, text and photo effects, digital painting, and layer and masking workflows. Individual step-by-step walkthroughs written by working designers, each usable on its own rather than as a course.
More resources on Adobe Photoshop
photoshopessentials.com
Photoshop Essentials is a tutorial site that teaches Photoshop through step-by-step guides and videos. It covers core skills such as photo editing, retouching, color correction, selections and masks, layers, and effects for beginner to advanced users.
Phlearn
Photoshop and Lightroom tutorial site with free videos and a paid pro library covering retouching, masking, compositing and colour work. Lessons are task-based, leaving you able to reproduce a specific edit on your own images.
Photoshop CC Bible
A reference-style manual covering Photoshop CC end to end: interface and workspace setup, layers and masks, selections, colour management, retouching, filters, type, 3D and video layers, and automation. Suited to looking up specific features rather than reading through.
Adobe Photoshop Classroom in a Book
Adobe's official self-paced training series for its photo editor, structured as project chapters with downloadable lesson files. Working through them gives hands-on practice in selections, layers, masks, retouching, colour correction and output, following Adobe's own recommended workflow.
The Photoshop Workbook
Six start-to-finish retouching and compositing projects from a commercial photographer, with downloadable images: character portraits, themed composites and landscapes. Shows the reasoning behind lighting, shadow and colour decisions, not just which Photoshop tool to reach for.