A lot of people will advertise their raids by taking a pic of their Switch screen. This frequently floods the channel out.
Raids can easily be condensed to 1-2 lines as follows.
6* Fairy Magikarp G0AWAY
Bring Perrserkers
#Disable image or media posting in raid channels
twin pebble
odd abyss
not to mention some of these pictures are wild 
camera phone from 100 miles away gotta squint and turn your head to get the numbers
scenic thicket
Some people (especially mobile users) find it much easier to snap a quick photo than to type out all the details. To each their own! I agree with you, however I think making it a requirement would be a mistake for the overall server.
twin pebble
