there are several methods to render an image
make sure each image has an alpha channel first
one method is to use the eraser tool and erase carefully all that is not needed
that method is kind of crude, but simple
another method is doing the same with the lasso tool or even more refined with the paths tool
so you make a selection around the object, invert and then delete
here is a tutorial for the paths tool
http://www.youtube.com/watch?v=TqFC1sSxF9Uthen you have all the tools to render an image: you have the fuzzy select tool, the select by colour tool and the foreground selection tool
for simple images with a lot of contrast between object and background, these can work great and quickly
check the manual for an explanation how to use these
a more complex, yet sophisticated technique is the use of layermasks
a very good tutorial can be found here:
http://www.youtube.com/watch?v=rRBXzVzwEoMthe really cool thing about using a layermask to render an image, is that its non destructive and you can always make changes and refine edges
finally there is the G'MIC filter Split Object, which is excellent and can be a very quick way of getting rid of the background of an image