I usually just say something like, "Welcome to Marie 1988 and enjoy your stay!" or something along those lines. I don't mind using the Welcome smileys, as long as it's not overdone, and as long as every last post doesn't have one. It's more personal if you type out your welcome rather than using an image.
I would congrats birthday or winning a competition with ordinary messages.. images can be pretty annoying especially when you take it from photobucket.. because when u don't use it no more they make this BIG BOXES about ur account exceeded limit or something.. it's just cluttering up forum imo. I don't against the idea though.