2
I can answer question number one.
There is an easy way and a hard way. The easy way would involve creating a duplicate webcam block, but instead of putting the webcams in there, you would put duplicate jpg or gif images.
You would then make the duplicate block visible to anonymous users only (it would be placed in the same position as the original webcam block), with some type of message advising them to register or sign in to view the cameras.
Next, you would set the actual webcam block to be visible only to registered users. Once a person has signed in, it becomes visible and the duplicate block is invisble to them.
Or......
You could do a whole bunch of javascript or php stuff that would drive a person to drink... well me, anyway. I'm sure there are quite a few people who could write something in a minute.
As far as question number two goes, check out this pop up window generator:
http://westchesterwebmaker.com/modules/xoopstools/index.php?func=POPUPIt may do the trick.
EDIT: Just took a second look at your setup. Continue to use the hyperlinks in the original webcam box that will be visible to registered users. DON'T use hyperlinks in the duplicate webcam box that's visible to the anonymous visitors.