User blog comment:SEOkitten/The Importance of Images/@comment-4129195-20151026232606/@comment-26321732-20151028162431

Our team would love to make it simple to add image descriptions on Special:MultipleUpload. Updating that field is the best practice, but it can be so tempting to just leave that section blank or promise ourselves that we will fill it in later and forget. . . as a contributor I am guilty of that one for sure!

File extensions do not influence image search or other search algorithms, but stick to file types that can be opened and rendered in a basic browser without special plugins. Our goal as contributors should be to choose the file type that is best for the image.