Re: 1) really depends on the copyright that Slitherine Corp is using. They very likely have a commercial copyright. TripleA is GPLv3. Simply attributing the images is not enough.
Re: 2) Uploaders generally assume any copyright risks. Copyright infringement is bad news "The law provides a range from $200 to $150,000 for each work infringed." Best case you're given a cease and desist, worst case you are sued for copyright infringement.
Unless you have written permission that the images are in public domain, I don't think you can nor should use them.