WordPress allows users to use some HTML in the comments. They can post a picture in the comments just like you would in a page you’re writing.
For example, using the following code when entering a comment would show the picture.
<img src="https://website/image.jpg">
Where https://website/image.jpg
is the URL to where their image is hosted online, for example their own website or image shack.
The second option, since most users are not very knowledgable with HTML is to use a plugin like the one here: https://www.trevorfitzgerald.com/2007/12/wordpress-comment-images/
(scroll down to the free version.)
It too, though, relies on the user uploading the picture on their own server somewhere, and then just placing the URL to the image into this plugin, which inserts the code into their comment to dispay their picture.
Hope that helps!