Another way of doing it, if the image is very irregular and it would be too time consuming to fiddle with the vector graphics, is to convert the image to an black and white image.
1) Go to the Arrow module.
2) Enter the part where you can manipulate images.
3) Select the button where you convert the image to black and white.
4) When the dialogue window appears, you select an apropriate threshold value. Everything light will be white and everything dark will be black.
5) This image you can then use as a mask on the original image.
This method demands an image with high contrast though. If everything is more or less in the same colour, this method can not be used. I have used the method successfully when I wanted to cut out a green plant from a white background.