In the Send-To menu there are KService item which are queried by "image/png" and kipi-plugins. The KService items could be (globally) configured by adding a menu item which opens the KDE Control Module "File Associations" with file type "image/png" preselected.
Hello! Sorry to be the bearer of bad news, but this project has been unmaintained for many years and I will be closing this bug. Spectacle is the replacement for ksnapshot now. Please test again and file a new bug for Spectacle if you still have issues. Thank you!