Just a word more. No need to use just the bmp format, .jpg files work too and they can be larger than 256 x 256. I've made ones as large as 8152 x 8152 for use over a large number of baseboards. Sizes must be a power of two. 256 x256, 512 x512, 1024 x 1024, 2048 x 2048, 4096 x 4096 etc.
That error in the manual was reported by me months ago but ignored as most things reported to N3V are these days.
William