potential spam and tracking images. The link is usually a call to a piece of JavaScript code that will replace the "src" attribute value on the "img" tags with the original URL. When the page loads, it usually filters the "src" attributes to load a semi-transparent gif image, hosted by your ISP.
An example of this would be if I had an image in my signature line that would load form my server. I would then be able to track every IP address that read my posts.
Since scripting is blocked on the DU servers, I cannot add additional specific information, such as your ID to the request...
Example:
[blockquote class="red"]< img src=" http :// someplace.com/someimage.gif?ID=wyldwolf " />
But, If I sent you an email and the contents of the message contained...
Example:
[blockquote class="red"]< img src=" http: // someplace.com/someimage.gif?ID=wyldwolf723%40domain.com " />
...then I would know that the spam message I sent you - 1) made it to a valid email address. 2) You read the email message.
Now, your address is added to a database to be sold to hundreds of other people trying to sell you Viagra, porn, Russian brides...
so, to answer your question... if you had the following HTML img tag, you could swap the src of the image like this:
< script >
function changeImage()
{
document.getElementById('8675309').src=' http: // someplace.com/image.gif ';
}
< /script >
< button ="changeImage();"> Can't see the Image? Click here! < /button>
< img id="8675309" src=" http: // your-isp.com/blank.gif " width="30" height="20"/>
Hope that makes sense...