Completely depends on the source.
Most of the time, if it says it's an OEM image it's fine. You have your own key and you'll be fine. I've downloaded OEM Windows 7 ISO's in the past to reinstall the OS on many PC's.
However, the hacked up, preactivated ISO's can be malicous. And illegal as well.
Sometimes you can get the ISO directly from Microsoft, or from their servers. I did this with Windows 7. Not sure about 8 though.
My question is why do you need one? Do you have a legitimate Windows 8 COA? Also, IMO, Windows 8 is a total pain, thus why I keep using 7 on all our PC's where I work. I have volume licensing for it 😀