"You must select an area first, before applying the crop tool."
Use the Box Select feature to select an area of the picture around which you wish to crop the background.
"Cannot rotate - picture will be bigger than 2000 x 2000 pixels if rotated to this extent."
When rotating a picture, certain angles could make the picture larger than the maximum allowed. If this message appears, either rotate the picture to a different angle, or reduce the size of the picture, then try again.
"Please enter an integer between 16 and 2000."
When resizing a picture, the largest size available is 2000 x 2000. If you enter dimensions smaller than 16 or larger than 2000 pixels, this message will appear. Choose a different size and try again.
"Not enough memory to save undo data."
This message may appear if you are using a large picture and your computer's memory or hard disk space is relatively low. Close other programs and try again.