If you have copied a fairly famous piece of art, no problem, fairly common practice, just list the original you copied. This place is pretty thick with "girl with pearl earring" and a bajillion things that are inspired by "starry night"
If it is a modern artist, who is not museum quality work, say your favorite anime/manga artist, then you should probably have the artists permission, and should at least have a link to the original in your description.
If you have trouble finding the original artist to credit or ask them for permission to post your copy, there is a way to 'photo search' on Google, and it's called the 'reverse image search'. You can upload the original image (or even your copied one, because I'm sure they look alike if they are indeed copied like you say) or paste the URL of the image. This will show you the locations of said photo online.
If you ever want to post studies and copies it is imperative you remember the original artist, either note it in the file name, or on paper or otherwise. It is just plain rude and disrespectful that you liked someones image enough to copy it to learn from it, but you didn't give enough of a hoot about the artist to remember their name.
Things like copies are the kind of thing that perhaps goes in an art blog where you're sharing sketches and WIPs, so long as proper credit is given and you make it very clear you were copying for learning purposes.
However if you were copying from stock photography, for example, which is a resource which is shared with the community on the payment of linking back, this CAN be posted as an art piece because the copyright holder gives you permission by making their work a resource. (so long as you obey their rules)