Probably the biggest usage of bandwidth on most normal websites would be videos and over-sized images.
If you put your videos on services such as YouTube and Vimeo, you will lower your website bandwidth requirements. You will also get the significant video management & processing features that are offered by those services.
Over-sized images are caused by uploading 'raw' images from digital cameras to your website. Typically these images are significantly too large for a web page and should first be web optimised, which makes them much smaller and also more efficient for transferring over the internet.
Many modern websites will automatically web optimise uploaded images, but will also still store the original over-sized image. Some websites will still send that large image to visitors and not use the optimised images even when they are available.
It is nearly always a good idea to web optimise your images before uploading them.
Do not use PNG images for photos when transparency is not required. They are significantly larger than the equivalent JPG/JPEG images.