You don't take screenshots with GraphicConverter. You take screenshots with MacOS using Command-Shift-3 to create a full screen image in a file called PictureNN.pdf, where NN is a serial number, on your desktop.
You use GraphicConverter or Photshop or whatever to edit the picture - crop it, resize it, annotate it .... and to save it as a jpeg or gif to upload here.
More optional info ...
There are alternative key combinations - include the Control key to put the screenshot on the Clipboard instead of into a desktop file. Change 3 to 4 to capture a selected area of the screen, but this doesn't work in Civ3 in my experience. Using 4 you can use space to select just the window under the cursor.