Optimizing images for your website is one of the major tasks for someone who really wants his site to have cool graphics but load quickly too.
So to speedup your page load time and to reduce the image size, they need to be optimized.
I tried photoshop to take off extra data from the png images, it works fairly good, but there are better ways to do that. Infact one of the web based optimization reducts the image size to almost 15% of the size generated by photoshop.
The site for optimizing png images online is infact image optimization tool by dynamicdrive.
I use it regularly to optimize images on my website (not just png, but gif and jpg’s too) and its really awesome. It also allows you to convert the image to other format (between jpg, gif and png) while optimizing it too.
checkout the options provided by them too! (planning to code a tool like this for nabtron too btw 0_o)