is this ok to do?

I was wondering about the legalities and stuff of using concept art off of the internet for personal purposes. Say I want to make a scene of this environment or whatever to get better, and I find this epic piece of concept art somewhere online. Is that okay to use that? I’d obviously look on the website to see if it says anything about licensing, but I just realized I don’t quite know what to look for. What if the site doesn’t say anything about it?

I’m unclear,

If you’re doing something trivially small or using the work in a way that can’t be directly associated with the source and are doing non-commercial work then I wouldn’t worry too much about it unless source specifies some very strict copyright.

If you’re going to spend a great amount of time to work on a scene then you can spend little time to find the author and ask him/her directly if it’s OK to use it as a source. I trust almost all artists would be honored by it.

I’m not a lawyer or anything, so no guarantees on anything I say. Using a piece of concept art and copying it to 3D is usually technically illegal even if just saved on your hard drive and never shown to anyone, that being said, it is incredibly unlikely that just having something like that on your computer is going to get you in any kind of trouble.
One thing you may want to consider is using several concept arts and using elements of each, modifying them to your own liking, making your own piece of artwork that you can probably do whatever you want with.

Licenses are typical divided as follows:

You are allowed to redistribute modified versions of the work
You are allowed to redistribute unmodified versions of the work
You are allowed to modify your own personal copy assuming that you are the only person who interacts with it (Can’t redistribute)
You are not allowed to modify the work (Must keep it as is)

I’m assuming the concept art falls under one the last two categories.
Even if the license forbids you from modifying the work, if it’s just for personal use, chances are incredibly slim that you will get in any trouble.

The concept art itself is copyrighted. Copyright simply occurs on creation and grants the creator the exclusive right for coping, distribution and adaption.
Styles and concepts can’t be copyrighted. They have to be trademarked. This does not occur automatically and you have to pay a substantial fee to register a trademark, especially for international trademarks.

Those two are facts and pretty much the same in each country.

So legally you should be pretty save, especially as you don’t intend to use it commercially. Morally - up to you. It’s just common courtesy to contact the person behind the idea.
That is assuming the source of the concept art is legal. If it is copyrighted and not published by the copyright owner or his consent it’s the first copyright infringement, and you downloading it to work with it, would be the second.

Then again, IANAL.