Ruination is on the ball there, do NOT use Image Extractor to convert icons to blp, you MUST use warcraft 3 viewer. That is beacuse w3v adds the required alpha channel to the icons, IE does not.
re: previewing icons in the object editor
putting the icons in the replaceableTextures\commandbuttons is not necessary anymore, that was a RoC thing. In TFT, once you import the icons with the import manager, it reads the icon image from wherever the icon is sitting on your hard drive. If you move your icons around on your hard drive though, you will lose the preview then and you must reimport to get it back.
The main reason to put icons in the replaceableTextures\commandbuttons folder these days is just so you can organise things nicely.