Sorry, this isn't much assistance but more a word of warning to let you know this can be very complicated.

Sending invitations via email can be complicated because the receivers can use a wide variety of email packages and devices to view the email. By default, graphics may be blocked requiring additional steps that the receiver must make to view the graphics.
It is certainly worth doing some tests of the possibilities and view on Mac/Windows/iPhone/Android devices to understand some of the variability at the receiving end.

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Andrew Lockton
Chrysalis Design, Melbourne Australia