How to add failed test's screenshots on RobotFramework Test Report?
From the Robot Framework plugin documentation:
If you have other artifacts such as screenshots that you want to persist for viewing later in the logs you must configure them under "Advanced... -> Other files to copy"
Click the "Advanced..." button under your Publish Robot Framework results post build action and put *.jpg, *.png in the "Other files to copy" box.
I'm pretty sure you can delete the first line "Directory of Robot output" or maybe put a dot in it. It's said that you have to give the path relative to your workspace.
For your screenshots issue, try to put screenshot.png in "Other files to copy", because images name are "selenium-screenshot-X.png" so I guess your problem is with the "/" you put in the regex.