I'm using Ubuntu 18.04, I took a screenshot which I saved as a PNG in my Pictures folder. Next, I right clicked the image and selected "Open with other application" to open it with GIMP. When I select the Paint Brush tool and try to draw on it, nothing happens at all. What am I doing wrong?
This is not remotely solvable because we can see only what you show and that's not much of your system nor of your GIMP settings. I have only 2 things to try:
- Reset tool options, You can reset all options of all tools to their default state with a single click
This is the general Preferences dialog. Resetting icon is also in Tool Options.
Do the same for input devices (see the main preference list)
- Uninstall GIMP and install the newest stable version