Do you upload images to a blog or website?
When you upload to a blog or website, compression is absolute key.
Firstly, always save as JPEG or JPG. JPG is the ideal photo standard for the web; as it is designed for fast transfer to the client PC and for a lower footprint on the web server. It works by taking out information that the human eye does not notice.
All images should be the minimum size that they can be before there is a noticeable difference or problem. For instance on our community shared blog, images for the top slider must be 662 pixels wide by 362 in height. If all images are uploaded this way, image file sizes become much smaller and therefore the web space (limited currently to 5GB) is cut down for this.
Another method of compression that I use is featured in my own software, Painter Pro. It allows you to select how much compression is applied to the image before it is saved. An alternative to Painter Pro that allows this is Paint.NET (another favourite) where when you save the image as a JPG it asks you if you want to compress.
Compression should almost always be applied to images, particularly now that mobile websites are a key part of any site.