Submitting to festivals... what content can be posted online?

Hi, I am planning to submit a recent short to festivals that focus on shorts. I know that for at least the majority of festivals, posting your entire video publicly is not allowed. What I'm not clear on is what content is okay to publish online. Is it okay to put a trailer online? (this is a short so there isn't really going to be a trailer) Can I post still images? Should I not post anything and pretend like it doesn't exist until I hear back from the festivals? This is something I haven't found a lot of specific information about.
 
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