Full size images with png on it… css

In each post with full size photo’s i have this code for one photo (i dont want to use gallery):
<img class=”alignnone size-full wp-image-[number]” title=”” src=”[image path]” alt=”” width=”533″ height=”800″ />

How can i do something like this:
http://img152.imageshack.us/i/20110211183432.png/

a png image over the full size photo… (png is something like watermark in bottom right corner)

i try this with css but its not working… i cant see this png file..
please help me 🙂