I think what I'm going to do is, have the promoter send a message to all the respective managers and just say, "Here's the website...If I've improperly used any promo materials here - please let us know and/or provide us with the approved materials."
The managers can't vouch for the use of random things (such as YouTube caps/links) gathered from the internet (unless they were the ones that secured the use from the original poster and are supplying that as promotional material). There is the "who gave the YT uploader permission to post the material on YT in the first place" sort of consideration but if that wasn't approved and the manager actually has something else they want used that's a double faux pas.
The way it is supposed to work is that the band/management should have at a minimum a press kit and photo (these days that should be an EPK and several images). If they post material on their website as an EPK or send it out as an EPK then that suffices to indicate they have the rights to use it and can assign those rights to promotional uses like yours (things on their website not explicity noted as for EPK or promotional use may not necessarily be secured for the use of sub-agents - see next post below for more about that).
Most managers actually want you to use their PK/EPK material rather than anything else. Some will insist you use those materials only. The further up the feeding scale you go the stricter it usually is though, as with most things, that's not universal.
If they don't have an EPK or press/promo photos to send you then unfortunately they're not functioning at a professional level (and should be gently told they need to do that because any promoter needs that material - they should after all be helping not hindering promotion of their appearances).
Your client/promoter should be tracking the materials down and supplying it, but if they haven't then getting the promoter a form email asking managers for images/content/promotional material/press kit for web development or asking the promoter to engage the contacts they booked the show with about this topic should get you clarity/materials in short order. Most booking agents will be sure to send it or include the links and any relevant stipulations in the contract for the booking (or immediately following striking a deal). Some of the smaller ones follow up a little later if they're booking a tour and send a mass mailer out to all the venues with a poster, photo, blurb specific to the tour...
Nice site BTW.