I use the website "piskel" to make my mapres, and that exports the right kind of png image, so try importing and exporting your mapres through that site
We with @Bomf are making solo map and can't choose a name for it, here's list of names if someone wanna help us
1. Grenade Galaxy (Rocket/Nade Galaxy)
2. Galactic Grenade (Galactic Rocket/Nade)
3. Cosmic Cannon (Cosmic Rocket/Nade/Grenade)
4. Star grenade (Star Rocket/Nade)
5. Lunar Launch
6. Nova Nade (Nova Rocket/Grenade)
Open lan server with your map -> go to editor -> do changes -> press ctrl+s -> open f2 console and type "reload", it will reload your map and you can test it