If you get black screen shots on Linux, please download and install the new version of Worksnaps Client here -- http://www.worksnaps.com/www/download.shtml.
Also, you need to install "gnome-screenshot" on your Linux system. Here is an instruction on how to install "gnome-screenshot" on Linux.
Execute the following command in a Terminal:
Install gnome-screenshot on Ubuntu
sudo apt-get install gnome-screenshot
Install gnome-screenshot on Fedora
sudo yum install gnome-screenshot
After you install gnome-screenshot, you can start Worksnaps Client as usual.
Please note that you will see the screen flashing briefly when screen shot is being captured. This is completely normal and do not worry about it.