You can't. Facebook doesn't allow pre-filled messages, per their Terms of Service. They want either a) no message at all with the action or b) the user to intentionally enter a message. That helps them determine the context and importance of the action for the user. You can read more on the Facebook Using Actions page. Specifically, for the message parameters:How can we have a default message in the pop up (the user may edit) while triggering an open graph action?
Allows users to write a personalized message attached to this action. You can only use this when the text is entered by the user and not pre-populated. You can mention users and pages inline using mention tagging.
I'll have to get back to you on this. I just did some quick investigation, and it's not a simple CSS line you could add for an image. I'll let you know what we find, but it may take a few days to get you an answer.We actually need that as part of an ongoing project.
It'll be great if we can create an image button for triggering an open graph action.
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