Look at the Steemd for this comment.

Unfortunately I think this challenge is _much_ harder than it appears if you want to get finniky with it- especially since some markdown parsers don't always work the way you expect them to. How do you tell the difference between say a [link](https://cdn.discordapp.com/attachments/404587860872921088/405329216280723458/challenge.png) that isn't really part of a post,  and an image? You can also escape the ! to make an image appear as a link. Example: \![link](https://cdn.discordapp.com/attachments/404587860872921088/405329216280723458/challenge.png)

Also - can't assume that the image is going to have a [valid extension](https://stackoverflow.com/questions/5110384/can-i-use-images-without-extension-in-img). 
For example, this is a link if I paste it in straight up.
https://steemitimages.com/DQmS9uzwXDM9HWNSM5G5EPhFd9VLi5Miva5MZdTPjcwL6rb/image
Now if I add the `![](stuff)` it gets interpreted as an image.
https://steemitimages.com/DQmS9uzwXDM9HWNSM5G5EPhFd9VLi5Miva5MZdTPjcwL6rb/image.png

I think the only way to come up with a solution that really works for all platforms is to render the page on Steemit and inspect that for valid image tags with something like BeautifulSoup. Anything else will only work 99% of the time ;)