You should try clicking the padlock icon at the bottom of the screen and then trying to resize the imported image, since you should be able to get a hold of it.. You'll need to try to click close to the black outline.
Otherwise, you may even be better served by resizing the JPG prior to import - normally, it's ideal to preserve as much resolution as possible, but there is a such thing as too big and it sounds like that might be the case, here.

Tim