Use this markdown syntax: ![image alt text for accessibility](link to image)
I think copy-paste works for linking the image.

For posting images, write ![Image](http://url/a.png)

I’m not sure how it looks on your instance, but on lemmy.world there’s a Formatting help button when posting a new comment:

pretty sure it’s ![image]() then in the parathes you put the link

edit: spelling