Hey! Try adding them the way this tutorial describes, however when you hit the tab for resources, add it to 'sprites' not textiles. Hope this helps! If not, let me know and try to specify what's going on.
You have to be careful adding sprits, some are just to big/small and look a little strange. As far as I know, there isn't a way to edit the sprites. You can however, if you google for some sprits and look closely, find the ones used for the default of the system. And if you know how (I don't) you could edit them from there.
Sorry! You may want to just keep trying, I'll post some sprites I've found that work pretty well for me sometime relatively soon.
On the main menu of RPGBoss editor, look on the upper right hand tab and hit the button labeled Resources. Once the page loads, you'll see a list of resources (No dur), and centered at the top will be two buttons, Import and Tilesets. Hit Tilesets.
Now your page shows you all the tilesets you have available, some have an x, a checkmark, or numbers on them. That determines their property. I've provided you a key down below.
x = Solid. Cannot pass through/walk on unless you've set up a vehicle that can. Checkmark = Passable. Can walk on/through unless you've set up a vehicle that can't 1/2/3/4/5 = The height of a tile. If a tile was '1' it would be a block high; this isn't particularly important except for the occasional roof tile.
I hope this was helpful! If you'd like to help grow this community, feel free to sign up and join the forum.
Tilesets are the best to work with since they are already made for these games - a simple google search for a tileset of whatever you need will typically yield results! As for adding them, check out this post.
If this doesn't answer your question, feel free to ask for more assistance, preferably in the help area.