When I was writing the post free wordpress themes I worked with some tools which used to decrease image size with out losing quality.
So here I will be sharing only two tools which I used to decrease image size.
1. Smush.itSmush.it optimizes images by removing unnecessary bytes. It is acquired by Yahoo and it is served under YSLow. You can upload bulk images at a time and all the images size will be optimized by this wonderful tool.
2. CaesiumCaesium is also a free tool to optimize image size. Like Sumsh.it Caesium also supports bulk image size optimization. But when I compared two tools by optimizing same images and I found that this tool is better than Smush.it in terms of speed and reducing size. So normally use Caesium to optimize image size. Both are good and both are free you can select any one. Download Caesium.
If you know any better image size optimization tool then please share here in comments.