I would install and use the Generico filter. One of it's included templates is a Welcome like you want that can be easily modified if it needs to be. I have used it for exactly the purpose you mention. The nice thing is that there are a LOT of available templates that let you you show/do many other things. Another nice thing is that you can place a Generico filter just about any location where you can open the Atto or TinyMCE editors.
Resource types
Automatic Popup in Particular Pages
This discussion has been locked so you can no longer reply to it.