Image Optimisation/Compression for Websites.

I was wondering if there's a way to reduce image sizes significantly for the Web. I've done my usual search on google, but unfortunately, I didn't find what I'm looking for. I'm still tinkering with Pixelmator, and the images are still too large. I've checked on many web developers websites, most of their images are in KB, and mine are almost 5MB to 6MB PNG files. Please help me out. Thank you in advance.