Images play a huge part in great website design however if it takes your visitors a long time to download the images many, especially mobile visitors will probably never be back. There are 2 rules to follow before uploading images to your website.
- Resize your images
- Optimise your images
For most websites, images should be no larger than 1000 pixels wide. If you don't have access to Photoshop you can use an online tool such as http://resizeimage.net/
Once resized, head over here https://tinypng.com/ to really compress your images down to the smallest possible size (often by as much as 60-80%)