Whether you want to just re-size the whole image or crop it to a certain portion, I would first set it so that the image aspect ratio is not preserved. There's probably an easier way to do this but what I do is select the image on the timeline and then go to "Keyframing". In there you'll find an "Aspect Ratio" box which is ticked by default so if you untick it then it becomes "squigeable".

You can now choose the crop tool if you only want part of the image or if you are keeping the whole image, whilst it remains highlighted you can just use the "grab points" in the preview window to re-size it as required.

Hope that makes sense to you (I understand it anyway ).