When working with websites - several common image types are used
One common image type is .jpg

JPEG stands for Joint Photographic Experts Group and was established in 1992
JPEG is widely used on the web as it supports millions of colours and a relatively small file size thus making it suitable for photography imagery on the web.

To achieve this file size jpegs use lossy compression which means data is discarded on saving - the compression level can be selected on saving - with higher compression meaning reducing image quality.

Avoid using a JPEG image as source/master image as continuing saving will see quality degrade at each save - also be aware this format doesn't support transparency

It is best to save JPEG as .jpg for Joomla Bamboo extensions/templates.