Publishing from what?
The Lounge Forum is not for technical help, please provide the name of the program you are using so your message may be moved to the correct program forum... A program would be Photoshop or Dreamweaver or Muse or Premiere Pro or ???
This seems less an Adobe problem and more like Flickr policy. JPG is a standard web image file format.
I quote from the Flickr website. This says it all. Flickr upload requirements | Yahoo Help - SLN15628