Just open the picture, chose "Picture" from the top menu. Then choose "Resize" for the drop down menu. On the right side you'll see a set of options. I downscale by 25% at a time until it gets too small and then go back up 5% or less at a time until I'm happy with the size.
Important, save the original under a new name, like "Pattern-Original", Picture Manager will work on the picture loaded unless you open a new picture.
Let me know if you have any problems.