You must obtain written permission from the proper and legal owner of any work which you wish to use, credit alone does not replace this requirement. Failure to obtain proper permission for the use of works protected under copyright can leave your deviantART submission vulnerable to being removed following a claim of infringement against it.
Explicit permission is not required when you choose to use valid stock resources. Please check your sources carefully to ensure that they are valid stock and please be certain to obey all rules, terms and conditions which may be attached to that resource.
If the work you wish to use is not a valid stock or resource material then you need to contact the proper copyright owner of the material prior to submission. Once contacted the owner of the material needs to provide you with a written statement which clearly states that you have permission to use the work and that you have permission to sublicense your derivative work to deviantART as required by deviantART's Submission Agreement. The owner should state that they understand the nature of both deviantART and the nature of the Submission Agreement.
For your convenience we have provided a sample permissions document HERE.
Other types of permissions statements may also be accepted.
Be certain to remember to adhere to any other conditions they may have given you when you submit to your gallery. For example; if they specified that you link to their website in the deviation's description then be certain to link to their website or your permission will be considered invalid and may be subjected to immediate removal. Also be certain to mention somewhere in your Deviation description that the necessary permissions have been included for review.
Please be aware that the proper copyright owner is not necessarily the webmaster of the site where you find the material. Webmasters are not considered to have the legal ability to grant you the necessary permissions to use the third party works which may be found on the websites they maintain.