Spam sent from your web site forms is annoying and wastes your time. theimagefile already does a huge amount to prevent spam from reaching your inbox, but with the Google reCAPTCHA option you should be able to at least know that all email sent from the contact form will have been submitted by a human.
Using reCAPTCHA is optional, and edited on a per-page basis on all advanced contact pages (under "Form Processing -> Spam Filter").
Using Google's reCAPTCHA requires an API key, and you need to list all your domains with your API key. We have a corporate key that handles tifmember.com, and so on. However for you to use it with your own domain name you will need to get your own API keys by registering with Google reCAPTCHA (and then keeping your domain name list up-to-date). To set-up your API key click the reCAPTCHA link next to the Spam Filter option on the advanced contact page.
Enter your domain name details on the Google "Register a new site" form
Click the Register button to get your Site key and Secret key:
Copy and paste these to the spaces provided in admin are on your account at theimagefile and save all.
If you have trouble setting up your Google API we have a workaround, with our own spam challenge (a math problem like "1 + 3 = ") The end result should be about the same but without usage statistics.