Image replacement involves using CSS to hide the text for an element (e.g. a <h1>), and setting the background-image for that element to replace it with an image. Users with visual browsers with CSS get the image; text-only browsers, bots, etc., just see plain text.

It’s not without its detractors and slight drawbacks, but it’s a widespread technique. A quick scan of big-name sites as of writing found it in evidence on stopdesign.com, mezzoblue.com and adobe.com.

However:

Hiding text or links in your content can cause your site to be perceived as untrustworthy since it presents information to search engines differently than to visitors. (Google Webmaster Help Center)

This obviously caused some panic among developers using image replacement. While Google seem to have made some comments saying that they would distinguish between legitimate usage and spamming, they’re pretty vague about what constitutes one or the other. The ever-informative 456bereastreet.com have a good summary. But while their conclusion is quite reassuring, it’s two years old - an aeon in web technology - and still vague:

While it’s good to know that sites are not currently being removed without a manual review, that could change in the future. So I would advise anyone making extensive use of CSS techniques that hide text to make sure that it can’t be mistaken for spamming.

How do you make sure? They don’t say.

More recently, SEO Critique took the approach of checking out high-profile designers with close links to Google and seeing if they used image replacement:

Rand Fishkin at SEOmoz does use CSS image replacement on the SEOmoz.org site, albeit sparingly. … I figure that Rand goes to enough conferences and has enough interaction with Googlers like Matt Cutts and Vanessa Fox that if there was a danger of imminent death by penalty he would know and would quickly order the offense removed. Hence, used sparingly and in the strict spirit of If a Blind Person Were Using a Text Reader How Would It Sound? My opinion is that using CSS image replacement is probably okay.

Probably not an issue to get panicked about, then, but one to keep your eye on. I’ll carry on using it judiciously until further notice.