I'm uploading this graphic here because it turns out I won't need it so soon (I was planning to use it for a project, but went with a total redesign). It uses SilverDeoxys563's modern Koopa as a base, but it walks on four legs to better use the cannon. The hitbox is weird because it replaces Snifit, which shoots bullets from its face, while this Koopa shoots bullets from a cannon on its back (which is located on top of the graphic). I don't know if there's a way to fix that. Nevertheless, it's an useful graphic for a lot of level themes. I colored the shell black to differentiate from other Koopa species, but you can change it to another color if you so wish. The graphical style is reminiscent of the All-Stars version of Super Mario Bros. 3.
width=38
height=40
gfxwidth=38
gfxheight=42
grabtop=0
jumphurt=0
playerblocktop=0
npcblocktop=0
I made it to replace npc-131, the blue snifit, but you can use it on any snifit to get your desired behavior.
I didn't make an effect file because you're supposed to use a blank effect file and make a shell spawn after you defeat the Cannon Koopa (using layers and events).
Here's a .zip download, if you'd rather have that.
