When all else fails
Go to
http://www.netmechanic.com/GIFBot/optimize-graphic.htm
Scroll down and click on "browse". Find your image, click on it and then click "open". Enter your "email address". Click on the "output format" and then click the little down arrow for your choices. Choose and then click on "test now" and wait for your image to appear. When it does, you can pick additional optimization options by placing a dot beside your choice or just leave it the way it is.
When the page loads with your images, scroll down and you will notice the difference in file size and byte size, etc. Also notice the further down the list you go, the smaller the file size is - but, also you will notice a difference in the colors, etc. of your image. Sometimes you can use this to get a great image and at a greatly reduced file size. But depending on the image, you might not be able to reduce it enough and still have any quality image left. You just have to try each individual image to see.
When you have one that you like, left click to bring it up on a page by itself. Then right click and save it to your computer.
IrFanview is a very good program, I have used it for years as a first step for website images BUT like all things there are better and more costly. Let me know if you would like some more help with anything..