Sorry to hear you are having troubles with Ninjaboard.
It sounds like a server related issue.
Could be depending on several things like upload restrictions, upload time-outs, maximum file size and such.
What I can suggest is to manage specific file types and upload limits within the Global Configurations under the System tab.
Also try to manage the Avatar settings within Ninjaboards settings corresponding with the Global Config settings.
If possible, it would be helpful if you could post a link to the page in question.
Also, if a login is required a temporary account (at registered level) would be great too.
This way we might be able to inspect things at closer hand.
Also, can you please post your system info here?
To do so, from the menubar in the backend go to Help > System Info > System Info (tab) > copy and paste the provided info.