Press the em2's shutter button or the Visor's Up button to take a Picture, then shutter/Up again to confirm that you want to keep the picture.  Tap No or press Down to cancel a picture.

Pictures vary in size.  Color Palm size photos are 38 KB, while Full size (VGA) photos vary in size.  My PowerBook image that I transferred to the Visor was only 66 KB, while most VGA photos that I took were from 130-200 KB in size.  My Visor with 1139 KB free showed that I could take 24 Palm size photos, 6 VGA photos, or a 3 second movie.  Since you can't throw in an extra roll of film, it is important to remove photos from your Visor once you have performed a HotSync.

In case you're interested, you can have the em2 on while in the Prism's cradle.  Perhaps some enterprising developer will make an application that keeps the Prism on in the cradle while streaming images from the eyemodule2 to a website.  American Pie 2 anybody?

Another consideration is battery life.  The em2 is rated at 1-2 hours of continuous usage.  My Visor Prism dropped about 10% from full after taking 4-5 VGA photos, so if you aren't using a Prism, make sure you keep a spare set of AAAs at the ready.

Lighting is the single most important consideration when taking photos with the eyemodule2.  Fluorescent lighting is best, so if you are planning on using your eyemodule2 in the office, you shouldn't have any problems.  The darker your surrounding, the more grainy your pictures will be.  Fortunately speckling is no longer an issue, but you will still need good lighting: