Avoid cross-posting to more than one list. Most subscribers are on more than one list and receiving multiple copies of a message can be annoying. The general rule is to make sure a message passes the four way test. Commercial messages of and kind are forbidden. Send an email message to the list owner.
Every message contain a header, which in the case of list messages contains two From lines.
One advises that the message came from the listserv and the other advises the original sender. Unfortunately, not all email readers show both lines. Avoid posting pictures and other binary data. You should first, advise the list of what is available and invite individuals to request the complete file. If the majority of a list should request the complete file, then it may be okay to post, providing it does not exceed the message limit.