Do you have a beach-scene graphic already, and you want to know how to display it in the background?
I would open something like Word, insert the graphic (if the problem is that you don't have a suitable one, get back to me), and then use Word's picture tools to fade it so that the text that you will overlay in Filemaker will be legible. Then copy the modified image, go back into Filemaker, do 'Control-L' to go into layout mode, and paste the graphic on to the layout of the Sales Order. Pull it out to be the size and shape you want. Use the 'Arrange' menu to 'Sent to Back' so that all the other text appears in front. (I would also use the 'Arrange' menu to 'Lock' the graphic once it is in the right position, to stop yourself inadvertently moving it when you are working on other objects in front of it on the layout.)
(If you're a whizz at something like Photoshop, GIMP, Photo Elements, or some other photo-manipulation package then use it, of course!)