Your Broker, flat fee listing or full service would have to agree to syndication to other sites. If they aren't doing it now they either don't know how or have made a decision not to participate (and reach the widest audience of homebuyers).
I don't know of any brokerages that would knowingly block this excellent exposure. Syndication from the MLS also includes many other sites like Realtor.com, other broker websites i.e. REMAX.com, Zillow.com and others.
In this instance it might be a case of limited service for a limited fee.
Best bet is to upload it yourself. If it cross posts eventually, you can always take one or the other down.
Monica's points are valid as well. You might want to ask the flat fee service if they have a relationship with Trulia, and if they don't, inquire as to why that isn't made more obvious up front.