Hello Felix505! Welcome to Wikitravel Shared. Please take a second to look at our copyleft and image policy, but feel free to plunge forward and add some images. If you need help, check out the help desk or post a message in the travellers' pub.
To address your question, that sea chart image is probably not a good idea to upload here. I know you contacted the owner and he said it was okay, but he really has to explicitly release the image under a free license, or release it into the public domain. You can't just pick one for him. I would suggest sticking to photos and other images that are clearly licensed for now, until you have a better grasp of the intricate legal situation involved.
I did add that link to your image from the Troppenmuseum, because Wikimedia Commons is where the free licensing for their images is recorded. We consider Commons a pretty reliable source for image copyright; if they say something is okay, then we usually feel comfortable using it ourselves. But the link to the Troppenmuseum web site by itself doesn't tell us anything about how the image is licensed; we need that link to Commons to tell us that.
Thanks for stopping by and for asking questions! Hope you stick around. =)
-- LtPowers 10:27, 27 July 2010 (EDT)