Just hold ctrl-alt-x
then you can draw a line around the thing you want. Then fill it with the bucket (the thing you want to cut out gets blue then) Just be sure to close the the line, else everything in the pic gets blue and you dont want that

The preview it, if you like it, click ok. If you dont, start over again. Its a great tool, works better then the lasso tool, but thats just my opinion

Greetz Daan