Adding a Tag, logo, subtitle, watermark or something

[drew something or other]

Though new to encoding and editing, between Avidemux and Media Coder I managed to bang out a decent video of a school play. I just need to add text or something that will identify it as property of the school. On this I am completly stuck. I've tried adding a .srt file but have had no luck. I'd love to add the school mascot but that is probably a bit much to ask.

Any suggestions?