There is in general no problem to hot link images or even to stored on other server if the material is used in non commercial way. It is good to mention source and give link, this give credit as well direction to other people where to search for more or ask for contact. As I know on ebay were used my color profiles as the promotion to sale some other products. As well the same profiles was not subject of sales, and they was hot linked from my site, I never take any action about this. I left to this person to make its income for life.
Cheers