Adding own background to Apple Watch faces

Hi all,

I would love to be able to add my own background images to my various watch faces. I realise this can be done using the “Photos” face, but I’d like to add to the Infograph and Modular faces.

I have seen apps that are able to do this. Well they say they are and the pictures shown on the App Store appears to show this, so I know it is possible. Is there anyone that can give me details of how the devs might be doing this?

If I need to re-sub to my dev’s account and use WatchKit it’d be really helpful if anyone here could give me some info as to how much dev work it would take please?