Thanks Steve for asking about how to crop irregular shapes in Gimp. It’s possible, though not intuitive, so I’ll break down the steps.
Let’s open up a test image and click on the Path tool (icon in the Toolbar). It looks like a pen.
Now click around the shape you want to crop. Don’t drag, just click the various points around the shape – a line will automatically connect these points.
When you get to the end of clicking out the path points around your shape, press and hold CTRL and click the first point to “close” it.
Now, we’re cooking because once you close the path you can turn it into a selection. That’s exactly what we need.
From the Menu bar, choose SELECT | From Path. You will now see the familiar dotted moving line around your shape.
Little tricky part here – to remove around your shape, you need to go back up to the Menu bar and choose SELECT | Invert to now select all the stuff around your shape.
Press the Delete key on your keyboard and *bingo* – you’re left with your irregular shaped crop.
Play with the settings on the Select before choosing it to get a “softer” cut line. Also, once you close the path, you can now even use the handles to arc and curve around your shape and really refine things.