This article has good information on image format choices for use in email.
I'm honestly surprised that animated gif is even in the mix!
PNG, GIF, or JPEG? Which is the Best Image Format for Email? | Litmus
February 26, 2014. PNG, GIF, or JPEG? Which is the Best Image Format for Email? [ 11 By Lauren Smith. With so many image file formats available—BMP, EPS, JPEG, PNG, and GIF to name a few—how do you determine which is right for your email? Each format produces variances in file size, compression, …
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