Thx Astx. Forgot about that function still being there, it was present in early firmware but i thought only for user testing. It's a 'stripped-down' version of the 34/35x0 App which also allows you the 'Show at Startup' and 'Set as Wallpaper' functions. The hidden 3597 function only lets you 'Delete' or 'View Slideshow'. Strange Garmin didn't add it to the Apps menu. Neither my NA 3597 or a friend's AU 3597 had the Pictures folder as OEM and of course there's no mention of Picture Viewer in the 35x7 manual [that i've found anyway] or indeed unsurprisingly no mention of the hidden menu behind the Speedo so it's not intended for normal users to access.

@Castus
On the map page, touch the speed field>Press and hold in centre of Speedometer for several seconds to reach the Diagnostics Page, then scroll down to and touch Picture Viewer. I haven't tried adding custom jpeg files, but you may have to respect certain picture size.