You can drag and drop images to Chatterino or paste them from clipboard to upload them to an external service.
By default, images are uploaded to i.nuuls.com.
You can change that in Chatterino Settings -> External Tools -> Image Uploader.
To reset that back to default, simply clear all the values.
Note to advanced users: This module sends multipart-form requests via POST method, so uploading via SFTP/FTP won't work.
However, popular hosts like imgur.com are s-ul.eu supported. Scroll down to see example cofiguration.