in my opinion this is a little strange question. For example, I always use a converter to download videos from YouTube.

I am a graphic designer and I often find interesting content that I download from youtube. Then I save frames from these videos and convert them from jpg to png https://img-converter.com/en/convert/jpg-to-png/  for use in my projects

It is not legal to use YouTube videos for any commercial purposes and derive any monetary benefits from it. According to YouTube one shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any content for any other purposes without the prior written consent of YouTube or the respective licensors of the content. So unless you have a written permission from both content creators as well as YouTube, your act of converting and downloading YouTube videos will be considered to be breaking the T&C & policies and potentially the law. Other than that, using YouTube content for commercial purposes can get you into a copyright, since most of the content on YouTube is copyrighted.

Can you recommend any converter? 

Hi, I can share some of my tricks. You see, there are a lot of such options there actually but not all of them worth exploring. If you want to save your favorite videos directly from YouTube without any quality loss, then check out some options of that great mp3 converter https://www.flvto.biz/en74/ I`ve already tested it and should say it works pretty well for me.