Optimizing images for the search engines depends upon many factors such as right format, type, file type, file size, caption, alt text and title text many more.
It depends upon the image if your image is getting rank for .png format, then you should keep that image in place and add .jpg format and optimize it accordingly.
For example, if someone is searching in Google "image" + .png and your image is getting ranked for that particular format then you should keep it over there.
For more info you should read the articles below:
How to Rank in Google Image Search
Image SEO: Optimizing images for search engines