Well, may be, there is no open source or freeware Photoshop alternatives, since the software is filled with complicated and sophisticated build-in features and external plugins. But, may be you do not need all these gimmicks, requiring long and detailed training and learning curve? May be, the simpler substitute will work fine, based on your exact needs and applications.
If you are looking for that, look at Krita – free creative sketching and painting application based on Calligra technology. Whether you want to create art paintings, cartoons, concept art or textures, Krita supports most graphics tablets out of the box. Krita’s vision statement is: Krita is a KDE program for sketching and painting, offering an end–to–end solution for creating digital painting files from scratch by masters.
Fields of painting that Krita explicitly supports are concept art, creation of comics and textures for rendering. Modelled on existing real-world painting materials and workflows, Krita supports creative working by getting out of the way and with snappy response.
Interface and Workspace
Krita has a user-friendly interface. In the settings menu, you can choose the color theme, which toolbars and dockers you want to use, and edit keyboard shortcuts. Save a given set of dockers as a workspace and switch between them. The canvas can be easily rotated and mirrored. The OpenGL canvas supports high-bit depth monitors. There is a large set of options available to create a no-distractions canvas-only painting mode.
It is easy to create seamless textures and patterns now. Press the ‘W’ key while painting to toggle wrap-around mode. The image will make references of itself along the x and y axis. Continue painting and watch all of the references update instantly. No more clunky offsetting to see how your image repeats itself. You can even paint off the edge and it will automatically start painting on the top.
Multiple Brush Engines and Blending Modes
A brush engine is more than just a typical brush pattern with settings changed. Each brush engine has its own logic and behavior. The included engines are pixel, smudge, duplicate, filter, hairy, hatching, texture, chalk, color smudge, curve, deform, dyna, experiment (Alchemy), grid, particle, sketch, and spray brushes. Brush settings can be saved as presets and shared. There are a staggering amount of blending modes available. The blending modes are arranged by category and have your favorites stored at the top of the list.
Advanced Selection and Masking Tools
Krita comes with many methods of selecting parts of your canvas in order to edit them. You can select with shapes such as rectangles and circle, paint your selection, polgon selection, select by color, select by Bezier. You can add, remove, or intersect to your selection. You can also make selection by layer contents by context clicking the layer and clicking “select opaque”. You can create a transparency layer by itself, or add one to an existing layer. This is great tool for non-destructive changes.
Symmetry Tools and Drawing Aids
Symmetry tools that go much further than basic mirroring. Take full control by being able to determine how many axis you need. Modify the origin center, angle, and smoothing parameters. Easy to toggle x and y mirror buttons in the top toolbar. Drawing aids such as perspective grids and shapes that have magnetic settings.
Filters and Effects
Filters can be used directly on a layer, or as filter masks or layers. The effect of a filter is previewed on the image itself. There are special effects like wave, oil paint, and emboss. Adjustments such as levels, brightness/contrast, and HSV are also included. Additional tools that can be useful for making selections like color to alpha and color transfer.
Layer and Color Management
Krita has raster, vector, filter, programmatic, group, and file-backed layers. Each layer has settings for visibility, edit lock, transparency lock, and alpha locking. Layers can be dragged and dropped to and from other applications. Vector layers support text, vector shapes and filters on vector shapes. Krita supports the following color models for creating and editing images: RGBA, Gray, CMYKA, Lab, YCbCr, XYZ in 8 bits integer, 16 bits integer, 16 bits floating point, 32 bits floating point. Krita always uses color management.
Download page (choose your OS): https://krita.org/download/krita-desktop/