In the case of educational and informative use, there should not be an issue with re-posting. Where the problem lies with copyright marks removed is when someone sees the image that they want to use for commercial purposes. Now they have no way of sourcing the image. Some photographers are very possessive with their work. That's why it's a good idea to ask first. I do know that almost all of the photographers that I personally know will gladly allow you to repost their pictures if you ask first.
Dan, if you see one of my pics that you want to make as a wallpaper, let me know and I will gladly optimize it for your screen to make it the best possible quality. Resizing the 750x499 for wallpapers looks ok, but when edited for your screen size, it pops! Plus, if you tell me something that you find appealing for a screensaver or background, that means others probably will to, and I can add that to my free desktop wallpaper section on my website.
While I am not ego-bound or anything else about my photos, some others may be. Source if you can. Trust me, taking beautiful pictures and posting them is a joy. Making money with them is very tough.