What Makes It Invisible
There are two things we need to do, to make graphic invisible. First off we need to use image editing software to make gif transparent. In other words, it takes on same color as background color of web page, which makes it appear invisible. The good news is, I've already done this part for you with sample you can download. The second part is to make sure BORDER="0" in html code. That will prevent a blue border or box from being drawn around your invisible pixels.
I wrote about this technique in my book "Nothing But 'Net" in section entitled "How to ensure a search engine can spider your pages." Its an old tactic that I've used for years. Word got out about cross linking and although this tactic is well publicized, few people have used it, not realizing how powerful and effective it is.
Cross link all of your home pages, internal pages, and hook pages to each other. When search engine comes to catalog your page it finds links to all your other pages. If engine is a spider it will crawl and add your other pages to its index. Being cross linked gives your pages a greater chance of being cataloged and having your pages "stick" in index. Some search engines even prefer this method of "finding" your pages rather than having you announce them to their submission areas.
Hope this clears up mystery surrounding cross linking and invisible pixels.
Michael Campbell Author, "Nothing But 'Net" How I generated $750,000 in internet revenues with no advertising costs. http://www.1-internet-marketing.com/nbn/