thank you for your quick feedback.
I have tried something similar to what you advise, splitting my 7680x1200 png in four tiles 1920x1200 each, and creating a transparent 7680x1200 png background (I'm using paint.net). After loading in Visionaire the transparent 7680x1200 background, it loads correctly the first tile as an object. However after loading the next (adjacent) tile, when trying to position it by the "set object position" button, I have first some weird behavior (both tiles not correctly displayed) and then an unhandled exception.
I tried again using a 7680x1080 transparent background (and 1920x1080 tiles) and got the same behaviour.
Of course my tiles are not squared as in your suggestion and also they are pretty big. I'm going to try with squared tiles next, but first I wanted to check with you what it would be an ideal size for the tiles (hopefully I won't have to place a huge number of them)
One thing I didn't mention before is that I am using the free version of Visionaire. Is the one on sale more up to date?