I tend to use Photobucket or Flickr as I have paid for accounts. Attachments tend to be too small and have less resolution but in saying that all formats should be encouraged as we just do not see enough images here in relation to other forums I use.
This comic one from last night. Flickr gives a good range of sizes and actually " enhances " the image in some way. The Flickr tag is annoying though.
JPEG stands for Joint Photographic Experts Group. All sensible operating systems will use either a filetype of JPEG, or append it to the filename. It is only the retardedness of DOS that foisted .JPG on the world because it could only cope with 3 letter filename extensions.
This is what Photoshop presents for saving files - it defaults to *.jpg when it saves it but if you explicitly type *.jpeg it will save it with that instead. I've noticed some apps won't accept one or the other though - took me ages recently to figure out what was going on - Affinity Photo defaults to *.jpeg. It's bloody annoying (and somewhat ridiculous) having two (or three, according to them) versions of a standard!