The best open source alternative to Corel Painter is GIMP, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to Corel Painter and 13 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Corel Painter are Krita (Free, Open Source), MyPaint (Free, Open Source), Glimpse Image Editor (Free, Open Source) and Seashore (Free, Open Source).
Photoshop in the browser? Photoshop ? Flattering, but not quite. BitMappery does however allow you to import photos, apply transformations, tweak filters and draw masks in a non-destructive manner. It has some overlap in functionality but is definitely a more simple product.