The most common formats for images on the web are JPEG (or JPG), PNG and GIF.
In most cases, JPEG/JPG is the format that results in a lighter image.
PNG is mostly used when you need the image to have a transparent background, which JPEG doesn’t allow. Like when using a logo, an illustration, etc. It creates higher quality images (because it doesn’t compress the images as JPEG does), but also heavier files. Therefore, it’s mostly used in smaller amounts and for smaller images.
Lastly, GIF is recommended for when you need to use animated images.
In conclusion, using JPEG will be the best choice in most cases. And use PNG when you need the image to have a transparent background.
In any case, you should optimize all the images you upload, so your site’s performance doesn’t get affected. Read how to do that here.