Sketch for Barter Stands

[LadyCricket]

Okay, so you can customize the sign above a barter stand by applying a sketch to it, correct?

But, question: sketches are 512 x 512 pixels, making them square. The sign above a barter stand is a narrow rectangle.
So if I want to make my own barter stand sign in an art program, what dimensions should I make it? Should I make a
512 x 512 canvas and only use the middle of it? Is there something I'm missing here?

And while we're on the subject, can you apply a new sketch to a barter stand if you're not happy with the first one?

[borka]

some hints

http://ringofbrodgar.com/wiki/Custom_Image_Sketch