I was looking at the backlinks of my competition and was impressed to see one site getting links from "very clean" pages that displayed no ads. To make a long story short: 90% of his links come from sites that link (sitewide) to a transparent image on his site. Human visitors won't see it but does it help with the search engines? All his pages have PR 4 and the sites that link to him have PR between 3 and 5 on all their pages. Example link: < img height="1" alt src="http://www. example .com/pixel.gif" width="180" >
It will NOT help but can get you into touble sooner or later! Even disgruntled searcher can complain against the site to Google. I had a similar incident where one of my friend placed him invisible link on my site's home page. Users wouldn't see the link but eventually his site was penalized and from PR7 he came down to PR3 in next update.
I agree. If you're going to do something simmilar, you should not use height=1 or width=1, a possible way is: <a href="http://www.anothersite.com"><img src="http://www.yoursite.com/img/onebyone.gif" title="TITLE"/></a> Code (markup): But still is risky, I think search engines are currently examining image properties. For example: images.google.com is filled with images taken from sites.