This is actually nothing to do with a plugin you're using. The default battleback is set in the rpg_sprites.js to 'Grassland'. You have a few options to get round this.
Pick a default battleback and change the name to grassland - this way when there's no battleback set on a map the game pulls this. This is the easiest method.
You can just set a default battle back on every map, this is the most time consuming method.
Change the value in the rpg_sprites.js file to the name of a different battleback. I could see this being prone to error (I personally don't understand everything that's going on in there).
Write or find a plugin that overrides the defaultBattleBack functions. Again, I can see this being prone to errors if you have to write it yourself, but someone else might already have a working plugin out there (might still have errors though).