When creating a new sprite definition,
you should have a dialog on the side with the image you selected,
and be able to rectangle-select the spot of the image you want to import, & make precise adjustments with the X Y W H editboxes :-)
Heres a screenshot I put together.. I got a little sloppy at the end because I got tired of drawing.. but you get the idea
(can't see this, but there is a "Preview" button on the normal add sprite dialog that is being covered by the one I drew, that makes this dialog on top pop up so you can select what area you want to add. This way you can go back & forth between the x/y/w/h & selection dialogs. :-)