When you want to make a copy of comic page, Simply select and drag-drop.
Hot key 'Ctrl+R' is for the rectangular selection, and 'Ctrl+E' is for the ellipse selection.
1. Select portion of page image that you want to export by Edit Menu > Select or Select Ellipse
2. Choose "Save as Image" from Edit menu or Ctrl+Opt+S for shortcut or simply drag & drop
If you don't select part of page, "Save as Image" menu will store the whole page image.