Without attacking you personally, it is wrong to assume that everything visible on the web should be free to use anyway you like. I myself upload images to numerous stockphotography sites (istockphoto, fotolia, dreamstime,…) and just because you can remove watermarks doesn’t make it OK to do so and use them for free. It would be of little respect to the photographer and clearly breaching laws that protect the copyright owner. Compare it to someone copying your masterplan design without your consent, or any form of compensation. I don’t think you’d be pleased with a minuscule caption stating you are in fact the architect. Additionally I know all to well that in the midst of things nobody will ever think to properly credit the photographer of an image you found on Google Images… And no I’m not a pessimist but a realist 🙂
While I had no problems sharing my photos for a very long with students and non-profit organisations for free through http://www.sxc.hu, I feel that professionals that (directly or indirectly) make money out of someone else work deserve to get compensated. Note that we are not talking about huge amounts but micropayments, often around 1-5$. In return you get a professional high resolution photo, you can help sell your project with. I don’t think it’s worth spending much time on trying to remove the watermark manually…
The websites I mentioned above refuse ‘non-commercial subjects’. Landscape themes are NOT commercial enough for the broad audience – expect for the very pittoresque – and therefor get very little coverage. This makes me think there potentially is a market for a specific landscape architecture / urban design (commercial) image library. Of course, nothing is stopping you from searching for free and 100% legal alternatives. I do so myself at times when my personal library proves incomplete. In that perspective I’d like to suggest http://doelbeelden.nl/ (sadly dutch only) and checking out the creative commons on Flickr.