Mel that is one thing I love about you, you are a lawyer to the end.
Some of us mind, some don't, most do not have any need to use the law to excuse what we are showing or how they do it. By definition the artwork itself may very well be in the public domain but the usage of the photograph of said artwork is more recent and I believe protected under current copyright laws. Now you return with fare use, etc., etc., et al and the argument never ends. This I will not argue any more.
The point is just make mention of where you get these things. We all love to have knowledgeable people on the site to lend their expertise in whatever area it may be and wherever the source may be from. Don't be a lawyer and be human being, when you post photos from someone else especially those from professional businesses who do not take them for fun but take them as a part of their livelihood and pay for that time/labor to do so and hide that photo behind a password protected portion of their company owned website which makes that photo not public domain give due credit for the fact of where you got it from because if that is the case then it has indeed been lifted. If they gave you permission prior to your posting the photo and sent or gave direct access to you then I heartily apologize for the lifted comment. Same as research. I have no problem with you using any information that you may want from me here or on your own site but once in a while it would be nice to have the person passing it along give that person the credit they deserve as they spent the time and effort to research their facts rather than make it appear as though it is yours or just magically appeared out of thin air. When you know whom the source is show them the respect for their hard work. For images many people here make posts with images from HA all the time, others with images from PM, EMP and others. More often than not they give credit to the owner of the pic or information.
As to how you got your PM images, I more than likely know how. I also more than likely know how you got the large non watermarked images from HA. The fact that you can do it does not make it right. So again stop being a lawyer and be a human being and respect peoples hard work and effort by getting their permission first or at least acknowledging the source. I feel the same about others that do the same. That is my point here and the last thing I will say about this.