When I upload images to my blog, I try to keep the file size as small as possible. Reducing image size helps improve the site load time. To grab screenshots I use Snagit and use Paint.net for any editing and resizing the images.
PNGGauntlet provides options to configure the PNG output. You can play around with the options for the best output. I use the default options and has been working fine for me.
Check out PNGGauntlet to optimize images for the web.