A quick sneak peek at a new feature in the next update - custom background canvas!
You will be able to select an image file or color and use that as the background canvas to your maps. See below to see the feature in action.
Notice how you can add the images/colors to the built in canvas picker. The canvas is then stored as part of the style settings and so can be saved as a custom style. If you have iCloud then the custom canvases (and styles) will be synced between iOS and OSX devices.
NB: Images are assumed to be repeating patterns. This feature does not enable a photo or logo to be used as a canvas (unless you're happy with it to be 'repeating')