I am not a lawyer, this is just my understanding, if you are really concerned about this issue, you should see a copyright/trademark attorney to clarify.
This is a tricky area. There is such a thing as "fair use," usually in writing, to quote a few words without written consent, many musicians use "sampling" - a few bars or words from another musician -- as for using in its entirety a photograph taken by someone else, I just wouldn't use it. I'd rather post my own photos, let them promote their own work. I sometimes make collages, but don't sell them. Even to cut out something like a cute bunny rabbit, well, that cute bunny rabbit photo is owned by someone else. I should look into it someday, how exactly does the law apply to collages that include parts of other people's photos. I mean, are we talking an 8th of an inch???
You probably don't receive any response to your requests because it isn't worth it for them to go after every person who uses their image, if you don't actually make any money off the image, you are in violation of the law, but unless the image is used in a way to denigrate the owner of the work, there are no real damages worth going to court over. If you made a huge amount of money using their image, that they would go to court over, though. If they went after everyone who posted something on a blog without permission, they'd go broke. There is no question who owns the image, they do, period. No reply is the same as NO.
I'm sure it's tempting as a design student to show images that inspire you, but you can describe your inspiration, describe what you saw that inspired you, and stick with posting whatever it is you were inspired to create.
As far as I know, no one has used my work, if they have and made any money off it, chances are, it wasn't much money and I'd never know. However, should they be successful enough to draw attention to my image and make money off it, I have unlimited resources to pursue the matter in court and have any profit from my image turned over to me. Believe me, the joke would be on them. What they'd have to pay in legal fees would cost more than the money they earned from my work.
As far as proof, anything I post on a blog goes through my computer. Under properties every image has the date the image was created. All of the Oldksool photos have the date stamped on the back. I seriously doubt I will find myself in such a position, but so many people have seen my work from such a long time ago, in progress at that, which I document by the way as the work goes along, anyone foolish enough to use my work wouldn't have a shot in hell saying my work is theirs.
I would find it completely disgusting and reprehensible if someone used one of my images without my permission, but other than that, not much harm done really. I recall a student in art school asking the professor what he should so if he found out someone was copying his work. "You should be so lucky," the professor commented dryly. That pretty much sums it up. We're not talking million dollar artwork here.
Speaking of tricky, as far as I know, anything publicly displayed is fair game. Your next door neighbor's house for example, but not the owner or the owner's children. If artwork is displayed in a window, you can take a photo of it, that's public, but can that photo be published? I don't know, as long as one doesn't claim to have created the artwork in question and doesn't make money off the image, I guess it doesn't matter. I once asked to take some photos of some really interesting fashion dolls in an exhibit, I was told no. They said something about how photos could be used to rip off the costume designs of the dolls. Okay, if I purchased one of the dolls, could I photograph it and publish it then??? I asked to take some photos of a cake in a bakery and was told no, so I took some shots outside the window, they have since gone out of business and I seriously doubt they will come looking for me, especially as I have never used the photo for anything.
I'm still not clear on the Andy Warhol using the soup cans. For all I know, he designed those. Maybe he didn't get permission, but it was such huge publicity for the soup company, sounds like they owed him more for free advertising than the other way around. You see how tricky this gets. I doubt Mattel wolud come looking for me, but I painted over a canvas I had painted a Barbie doll on, too bad, it was a cool picture, I was never going to be famous enough for it to ever catch their attention.