The images need to be somewhere online, yes. The easiest option is to use Dakka's Gallery -
https://www.dakkadakka.com/gallery/
You can upload your pics, and the gallery software provides handy copy-paste links to post the images to the forum or elsewhere in various sizes.
Alternatively, you can use the Attachment function on the 'New Post' screen to upload the pics directly when you post - any pics attached to posts in the Painting and Modeling sections of the site are also automatically added to the Gallery.