You should just be able to insert the image URL directly to the URL field (for posts) and for comments or post bodies you should be able to use markdown formatting to achieve that. For example if you enter ![](https://static.pexels.com/photos/45201/kitty-cat-kitten-pet-45201.jpeg)
it should render the following image inline:
The format for embedding images in markdown is ![optional_description](image_url_here)
.