how to stretch a single wallpaper image across multiple monitors

Solution 1:

Your picture actually is the perfect example of why this is incredibly difficult to do automatically.

You have 3 totally different screens [& for this the actual distinction as to whether one is a Retina or not is moot]

Visually you have one small, one large & one in portrait.

Bear in mind this is extremely rough...

If you divide the picture into 3rds, one third to each screen, then you get this...

The left monitor has what was a 'tall' ⅓ now squished to fit the screen.
Same for the centre monitor.
The right monitor has it's ⅓ give or take, about right.

The alternatives would be to eliminate any portion that doesn't fit,

or shrink it & leave blank space around it.... (but I'm sure you get the idea by now ;)

Really, the only way to do this & have reasonable control over it is to do it manually, then set each portion to each screen.