You can't, unless it says it's FTU or F2U (Free to use). Although if you're wanting an image straight off the internet, you can't just credit it because the majority of the time that image was posted on another site instead of the original one.
You can make your own avatar or get some art here/other sites! To credit your avatar if the artist is on Wolfplay, you can just put in their pack number! If it's somewhere offsite, such as Toyhouse or Deviantart, just put the person's user and add the social media type!
If it's from another sim site, avoid using it, and try to find the artist's other social media accounts!