Let me respond to both posts at once. Yes, the image is public domain and not copywritten. However, someone, somewhere has to PAY to keep that image on the internet. You have a Facebook account (for the same of argument - I happen to not have one), they make money off of selling advertisements to you to justify the cost of hosting that image. The owner of Navsource comes from a long Navy family and identified a need nearly 20 years ago - he pays for that site out of his own pocket to keep those ship histories and photos online. Regardless of copyright or public domain, somehow that image is being paid for.
So, I'm not saying you should not embed images in web sites - it's a very powerful tool. I'm just saying consider the web operator in this equation. if the photos are copy written, then there are legal issues with you saving and then "re-homing" an image so that you can - it's best just to say something like "you can find a drawing here." Less chance that the owner will find out and file a take-down notice that will break the post in the future and possibly get you in some trouble.
If it is *not* copy written though, the best thing to do is to save a copy and upload it to a free image hosting company such as Imgur or PhotoBucket - they will then give you a unique URL for that image you can paste in to a post to in effect do the same thing, but it on THEM to pay for it now and not the other operator. Best-best thing to do is re-home and then say "I found it here" so the web site operator still gets a nod.
It adds a layer of complexity to things, for sure, but like I said, we almost lost two good websites because of it and it affects countless others. It's not something that gets taught with any "internet basics" course, but it is a "good citizen" thing to do.
But no animosity on my side.