You need an image editing software to do that. You'll need to edit the image so it is 2 tiles, half on one and half on the other (note you can use other ratios, but half on each will let you mix and match better too in case you wish to use other barrels later).
I'd recommend also saving the edited version in a new tileset, so you still retain this one. All of my edits I put in my D tileset tab too.
Using a software and using images for maps like bgillisp suggested is done through a process called parallax mapping in RPG Maker. Go check out a tutorial on the subject, and there are good plugins to achieve that.
Working on a Premium version of my minimap plugin that will have advanced functionalities such as drawing a Terrain map based completely on the users tilesets, whatever they may be, and drawing minimap radar icons automatically based on texture of the event the note is applied to.