Having images on your website pages makes them more interesting and is a great way to showcase your services, your work, or your products. However if you're not careful you could cause your website more damage than benefit. How? If you don't resize and optimise your images, and assuming that they have come straight from your camera or mobile phone camera, then they will be in their raw format and quite large. By large I mean both in their physical dimensions eg 3500pixels wide x 2000 pixels high, and 4meg in filesize. Let's say that you were uploading an image for a gallery which also had a 'click to view in lightbox' type of functionality. You would probably only need the image at it's lightbox dimensions of around 1000pixels wide x 800pixels high, and not as such high resolution as what your camera took it at. Web graphics are usually at 72dpi (dots per inch), or for high resolution screens 96dpi.
Resizing and compressing an image will drastically reduce the filesize of your image, which will mean that your webpage is faster to load than if you had included the raw image file. For example, it will be much quicker to load an image which is 50kb rather than one which is 4meg (4000kb). Imagine a page full of 4meg images, 10 images could easily become 40meg, and with the huge amount of mobile traffic it will not only be slow to load your page, but will chew through mobile users data too!
Free Online Tools
Here are a couple of free online tools that I can recommend. I spent some time comparing online tools and these ones gave the most options and the best compression:
My suggestion for a free image optimisation tool is https://tinypng.com/
My suggestion for free image optimisation and resizing tool is http://resizeimage.net/ the only disadvantage is that you can only process one image at a time. I uploaded a Ford image and set the 'Resize your image' to 800 wide (auto filled in 600 height), and set Optimize your image to ‘Normal compression’ with value 75%. The result was: size before 2500kb, size after 57kb. That’s a massive difference, with no loss in quality!