Board Thread:Support Requests - Getting Technical/@comment-26015755-20150509123521

I wish to add images to Wiki pages, but I am not sure about the copyright issues. I read the page "Help: Image copyright tags", but failed to understand it. For example: if I take a screenshot of a YouTube animation clip, then edit it, leaving only the part I wish to add as a photo to the Wiki - what are the steps? How do I know which licensing tag to use? Do I have to ask permission from the one who uploaded the clip? Please answer me step-by-step, that I will not violate any copyright by mistake. 