Pick a few favorite images and try to find out who created them (a large percent of them should have some kind of signature: look for it either on the image or somewhere near it (in its title etc.) For unknown painters/photographers and images, try TinEye.
Once you've got the name of the painter/photographer, google it (not the theme, as you say in your Q.) Finding the official portfolio site of a given artist is possibly the best way to find images of the same style and theme. ;)
There's also the option of going to art community sites that host thousands of images in galleries, and are, in most cases, searchable by tags/keywords. Examples include: drawcrowd.com, cgsociety.org, conceptart.org, the better galleries at deviantart.com (there's been too much "noise" on dA lately), and so on.
Another way to look for stuff is to (register and) use the search function on sites like tumblr.com or pinterest.com. Look for specific artists (as I've recommended above), or for themes and topics (like "forest", "monster", "d&d", etc.)
Note, please, that the results you get from these sites are practically never "stock" in the "you are free to use / republish / etc them anywhere online or offline" sense. The images are copyrighted works of their respective creators, and though you're free to show them to your gaming group on your device, you have no rights whatsoever to them, so don't use, republish, etc them in your publication, blog, etc, even if you're not doing anything commercial. To do that, you'd need to consult the licensing guides of the sites and the respective artists (and pay them.)
Sure, tumblr and pinterest are full of reblogged/repinned content, but it's a rather gray area of the net. A lot of artists and sites welcome and encourage sharing this way - going so far as to implement official buttons that help you do so -, but it's not true of everyone at all. Find, read, understand and follow the licenses before spreading art, that's the safest thing you can do.