About the images, on my server and last version of the plugin, all images are loading ok, maybe try a server restart?
And about the Success Chance Per Level this is really a good idea to implement directly in the Admin UI, and I think I can make you be able to take the upgrades to infinity but it will be an experimental option that can be activated from the admin ui, I will test some methods for these things, I will come back with them in the next update! Thanks for the idea! And for now stay at 100 levels, it is possible that the plugin will break if you go over them without modifying the other calculation formulas. You will create weapons that give a bullet and destroy entire bases :)) or... give it a try and test, I am curious what comes out! until I release the update with these things, have fun!