I was working with a client the other week who had quite a few large .png images loading on one page which made the page consume over 2MB of load data just for a single page online which kept the Google Pagespeed scores a bit lower. Naturally without getting a CDN service you can compress or shrink png files and save a little bit of space and pageload time.
In case you don’t feel like converting them to .jpg which take up way less space than a PNG but would require updating all pages that call upon those images, you can leverage a freeware utility like PNGGauntlet to automatically compress your PNG images.
PNGGauntlet is free and you simply define an output folder, add all your images via drag and drop or manual selection and just click Optimize.
This will go through every image and compress it showing you the original size, compressed size and the total % saved by compressing the image. When I compared the images side by side in Paint.NET I was unable to see a visual difference in image quality after compression so it works pretty well.
Download PNGGauntlet now if you want a freeware program to compress PNG images for Windows 7.
If you are looking to compress images on Mac OS X, then try ImageOptim
If you are looking to compress images for Linux, then try Trimage
Web Based PNG Compression
For those who don’t want to download a client and are looking for a website that lets them upload png images to compress, you can find this with PunyPNG.
Simple and easy to use, you just hit PunyPNG website and click on the big green upload button.
You can upload up to 15 files each with a max size of 150kb with the free version, the paid version allows up to 50 files at 500kb each. Either way I found that when uploading files in bulk it worked for 6-9 files then crashed Chrome browser or kicked back an error message, so I would go with the PNGGauntlet freeware which has no file size limitation.
Either way these are a few options to shrink png or compress png files that are too large and you are looking to speed up pageload times just a bit. I found Smush.it to be unreliable lately I was using this for all of my images previously.