This functionality is something that I am potentially going to explore, but it would be fairly involved so it will be further down the road before I look into the possibility. This also strays a bit from the voxel style of the game which I am not sure I want to do, but I am not ruling it out as a possibility. :)
I am for sure planning on adding a pixel editor that will let you create images for things like items. I have a very basic start to one currently, but only for a specific case - if you go into the UI Editor tab an click Edit Image for the dialogue box appearance, you can check it out. I will be building upon that and eventually have a separate image editor.