This might surprise you, but not all image files are created equally. Programs that save images often do so in an inefficient way. Thankfully a lot of smart, dedicated people have built utilities that correct this.
If you are on a Mac, install ImageOptim. On Windows, install FileOptimizer. They work essentially the same way. You drag an image over (remember to always operate on a copy), and it runs a bunch of different utilities to compress the images without any loss of resolution. It’s pretty impressive how much you can save. For simple illustrations and icons, I’ve seen savings of up to 80–90%. More typically is in the 10–30%. But that definitely adds up. I’ve compressed entire folders of raw images from my phone and digital camera to save space before I save them into my photo archive.
A quick note on file formats. For real-world photographs and realistic paintings/illustrations, JPEG is the format to use. For logos, maps, and other illustrations—especially those with a limited color palette—use PNG. I don’t recommend GIF, TIFF, BMP, PICT, or any of the legacy file formats anymore. These are the two best and most widely supported.
If you want to reduce the size and don’t mind losing a bit of quality, then get an image resampling app like ImageAlpha. It has a live preview, so you can see instantly how the new file with a reduced color palette will look. In many cases, it’s difficult to see much of a difference and you can save a lot more space.
And for cases where you don't care about quality at all, I use the most excellent Mac droplet Make JPEG. It’s quick and very useful for significantly reducing a full fidelity screen capture to a workable file you can send over email.
Finally, there is a great web-based utility for optimizing PDFs called Small PDF. A word of warning: it will significantly degrade the quality of high-resolution images in the file, so don’t use if you are planning to print.
So why do we care? Because speed matters. You want people who are viewing your web site or email messages to see your content as quickly as possible. This is especially true for mobile devices when network speeds can be a lot slower. Google is one company that really gets this. Yes they have high-quality search results. But part of the value of their product is the speed. To this day they still show the amount of time taken to return a search. You might not notice it, but it’s always there right above the top result (and the ads). Back in November Google Research announced a very clever tool that allows you to scale down images to ¼ their original size and then use software on the other end to regenerate the details. I'm looking forward to being able to use that on our web site to double the speed.