Well, tubes are fun and pretty straight forward.  When you click on the Picture Tube tool, you need to also have your Tool Options window open.  Then it's a simple matter of choosing which little tube picture you want to use.  Other than that, there isn't much new to tell here.  Oh, I highly urge you to add a new layer before adding a tube picture.  That way you can move the little image around.

One interesting thing you can do is hold down the left click and move the mouse around to create an interesting look with the tube.  Tubes like the 3D's, Rope, Cosmos, etc., work well doing this.

You can also customize one of the standard tubes that come with PSP7.  The Cosmos tube normally has a Step of 300.  I changed it to 100.  At a Scale of 25, I held down the left click and moved downward.

What if I wanted to save this custom setting?  Here is the method I came up with.  Go to the Tubes folder and Copy the Cosmos tube.  Now paste and rename it Cosmos2.