I spent a couple of months developing a Unity extension that made it possible to edit images directly inside of Unity. As long as the image size was less than 512×512 pixels, everything went smoothly. If the image went over the 512 limit, Unity would begin to freeze and stutter, and the extension would become unusable. It may have changed, but back then it was clear that Unity was not designed to handle things like image editing.
The project got too big for a single person to develop and maintain, so when I lost the source code after a big backup failure that cost me one year of work, I decided to discard the project.
I still have the compiled DLL files for the project. If you want to try out the extension for yourself.
Warning: This is just a prototype. The project is compiled for an old version of Unity so some features do not work correctly anymore.