Currently, I send PR related invitation by sending email directly to the mailing list id of my group on meetup.com ... However, in the message boards and many responses from members, they say, that they did not receive any image in my messages :( ....
And to log in to meetup each time to send a PR related email to all, by uploading an image to meetup first and then sending.... is quite cumbersome.
fyi, I am already the co-organizer of my meetup.