Pictures in meeting mail

Hi,

Meeting invites have specific format.
We struggled to have these emails identified as meetings in most email clients (Outlook, Gmail, Thunderbird, Zimbra, SOGo).
With Outlook client, a single unexpected character leads to email not treated as meeting
So we don't expect to send full HTML fileds in the invites (in fact I'm not sure all clients would support this).
Ticket #5221 recorded to see if we can manage somethong, with very low priority.