You can use your image instead of default by dragging and drop your image into main working image area.
You can select clipping area in two ways: Lasso or Polygonal lasso
To lock clip selection with Polygonal lasso, you should click second time on last marker or put next marker near the most first marker
You can disable image and use background color. To change background color use color picker tool
You can easily reset selection
You can hide/show markers if they interfere
You can disable/enable clipping, in this case there will left only selectiong border
You can add new marker into selection by clicking on edge when crosshair cursor is there
You can select any marker by just clicking on it
You can change selected marker position by dragging it across area
You can edit lef/top position for selected marker. Also you can remove selected markery by clicking on Trash button or by using DELETE key on keyboard.
To navigate between markers you can use Next/Prev buttons in marker section
You can move you selection in place you want by dragging