Is this an infringement of copyright?

Yes, it is infringement. It doesn't matter if you are not making a profit, selling them or that you got them from someone else who also infringed on the copyright. You can still be sued for up to 150K per image/per use.

Follow fhotoace's instructions or go out and find images licensed under Creative Commons. These are free to use provided you don't use them to make a profit in anyway, e.g. ads on your site.

Copyright remains with the photographer unless s/he gives you express written permission or assigns the rights to you in writing. Fair use is generally limited to educational use - you used it for a class project. This could be expensive to prove, so why have that problem? Get stuff that is licensed as CC and you're good.

There are a number of legal issues. Most countries have some type of copy write laws that cover originally written material. You are talking about photos which definitely were owned by who ever created them originally. However, you are also talking about the internet. I would suggest that you give credit when ever possible. but there are times when the origin of photos cannot be determined. This is a gray area and you will only know about an infringement if someone takes you to court , which almost never happens from things taken from the internet.

Personally if I was really concerned I would contact an attorney.

Yes, it's probably copyright infringement.

However, if only you used the images only for comment/editorial use, then you might get away with it. You could claim fair trading (called fair use in the USA). If you are making money from the images in any way (commercial use), then it's totally illegal. Do you have any advertising on your site? If so it could be viewed as a commercial site.

If you have any problems, or anyone asks you to take the images down, you should comply.

Whether you profit from the use or not makes no difference. You must have the permission in some way from the original owner of the works. It would be like taking something from the store and telling the clerk you will give the store credit for it (which nobody pays attention to anyway).

That would be illegal.