You need to use the image address and remove the spaces like this:

![PoW vs PoS](https://nitter.net/pic/media%2FFZVdzXlWQAISpPv.jpg%3Fname%3Dsmall)

https://nitter.net/pic/media%2FFZVdzXlWQAISpPv.jpg%3Fname%3Dsmall

Just that you know :)