For your security and anonymity.
DO NOT UPLOAD JPG/JPEG IMAGES!
ESPECIALLY IF YOU TAKE THE PICTURES WITH YOUR PHONE!
Your phones can and WILL embed your photos with information that really doesn't need to be shared. You are basically fucked if you leave your GPS on. As it will embed your location when you took the photo which can easily be googled by anyone which includes LE.
You don't even need to convert to a *.png. All you need to do is remove the metadata. There's one built right in to Tails called MAT and it's located under System Tools.