Hi,
You actually don't need ReefMaster to create background images for your Lowrance. You can use the Insight Map Creator directly, which is a free download from Navico, and create a raster layer which you can then view on your unit by selecting the shaded relief chart option.
All RM map exports to Lowrance go via the IMC but we don't actually have a function that helps with putting arbitrary images onto maps.
Matt