There is not really a good answer to this question, as the image format must be determined according to the type of content you are broadcasting.
Do you want to integrate an image into your email because it contains a lot of text? Use the PNG or GIF format.
Do you want something animated to stand out from the crowd? Use an animated GIF.
Want to highlight a very colourful image for your latest product? Try JPG.
Want to create a new minimalist logo? Then a GIF or PNG is right for you.
The most important things to keep in mind are the appearance of the final rendering and the size of the images.
If you add too many good quality images to your email campaign or several animated GIFs, you may get a great result, but it may take a few seconds to load, which will reduce the way your subscribers interact and increase the bounce rate of your emails.