Technically yes it is an infringement. The strange thing is that in law inlining the image in a page is considered less serious than if you were to copy it over to your server and host it from there - even though when inlining not only are you breaking copyright but you are also sending them the bill for the hosting!

In reality if the page is not in a restricted area (i.e. a payed for service is being provided to others without them paying), and its fully obvious where the image has come from, its unlikely anyone will have a problem with that. But in general, ingoring a few cases that could possibly come under "fair use" type situations, its an infringement of the copyright if the owner has not specifically stated that content may be reproduced.