Green Mage

Don't worry at all, as long as you're not making any profit out of it, and always remember to credit the original creator( link back to original art work, state the name or the artist and clearly state that this is a copy of it) you will be fine. Most artist start from copying other people's work, from their favourite anime or manga, or their favourite artist etc. that's where they learn and later on start develop their own style.