1. Go to Pornpros.com

2. Select yout favorite Video via "Our Girls" or "Latest Updates"

3. Go to the Detail Page of this Vid

Example:
http://www.pornpros.com/episode/?episode=3387

4. Look at the URL of the Big Sample Preview Pic
In this Case:
http://cache1.images.pornprosnetwork...pg-299x372crop

5. This is maybe a bit tricky:

You have to build your own url to access the protected content via the Cache servers

http://cache1.images.pornprosnetwork.com/cms_data/ *SITE-NAME* / *MODELNAME* /videos/ *MODELNAME* + "-wmvFullHigh-1.wmv"

So for the above example it would be:

http://cache1.images.pornprosnetwork...FullHigh-1.wmv

Or another example:

Detail-Page: http://www.pornpros.com/episode/?episode=3152
Pic-URL: http://cache1.images.pornprosnetwork...pg-299x372crop
So your URL has to be
http://cache1.images.pornprosnetwork...FullHigh-1.wmv

Additional-Infos:

The Folder has to be "/videos/" instead of "/photos/" in the URL if you change the Preview-Pic-URL

There are a few more options:

as last part of the URL you can choose between

"-wmvFullHigh-1.wmv" WMV-HD (means not everytime HD but imho the better quality version)
"-mpgFullHigh-1.mpg" MPG-HD
"-wmvFullLow-1.wmv" WMV-LOW
"-mpgFullLow-1.mpg" MPG-LOW