I added some temp graphics from Link to the Past to make development a little easier on the eyes. Anyway, this form is pretty self-explanatory, it allows you to change the graphical properties of a tile.
Because an extra form means extra clicks and I wanted to preserve efficiency for basic cases (simple colors, 0/25/50/75/100 opacity, etc), I created a convenient right-click context menu on the texture selector to let you quickly pick and apply properties to the tile instead of opening the form. Currently the 'texture' submenu loads (lazily, at least) all textures, shown below. When the list of textures gets larger, I'm thinking of implementing some sort of 'path' organization to the texture set editor, and that path will drive a submenu structure (for example, Textures -> Outdoor -> Grasses -> Grass 1). For now, the flat list will do.
Since I'm working with layers, I figured it might be convenient if I were to be able to use the 'drag-style' copy and paste to quickly fill an area with, say, grass. The problem is that currently copy-paste (and 'drag', which uses the same paste implementation) was copying all tiles, so if you wanted to change one of your ground layers quickly (say, from dirt to grass), all of the other decorations would get wiped out. To solve this, I implemented the 'Copy Layers' menu, shown below, which toggles individual layers to be considered part of the paste operation for tiles.
I also implemented the display of transparency, horizontal flip, and vertical flip. Added 'custom' to the background and grid color selectors on the view menu.