Why does this PNG show differently sometimes?

I am playing around with CodePress for a small project, and I am trying to figure out how it generates the actual numbers for the line number display.

I have looked all through the code, and I see nothing where 1, 2, 3, ..., n is outputted. However, when viewing the page info in Firefox, I see this: alt text http://grab.by/grabs/3bce2f7860459dabcf4019862ec320a0.png

Now, when I actually go to that URL in Firefox (here is the image from a CodePress example site: http://bobdawg.org/codepress/images/linenumbers.png) I just see a line with no numbers, like this:

alt text http://grab.by/grabs/11cbd4cd7808ebccde0de59a1b0开发者_运维问答a63b5.png

So how in the world are the numbers being generated? Clearly they're somehow embedded in the PNG, but how?

The image dimensions are 26x24010. What you see in Firefox is a zoomed out version of the image. If you click on it to view it at full size it will look the same as the preview.

You need to zoom in.

Save to desktop and open with Paint.