The main thing is not distracting people. If the images help illustrate the point you're making and meanwhile you're keeping the audience engaged they work great.
If they are busy and distracting, they will stop listening to what you are saying and focus on the image - it's all about finding the balance, and using your skills as a presenter to complement that too.
Pictures are potentially a great tool to make your presentation more engaging and memorable, but the last thing you want to do is have them not listen to what you say!
The watermark may be useful when sending out your presentation after the fact, but if it's during a presentation, it may just become distracting - certainly if it's hard to see, people may focus their attention on trying to read it, rather than listen, does it really add to the presentation having that extra visual noise?
Hope that helps a little.