There are 3 ways to post works within a group.
(1) Post your work to pixiv and then mention the link in your group
Works on pixiv have a unique URL you can post. The thumbnail for it will show up automatically when you post the link.
(2) Post your work via the "Add images" within the group
Once the work has finished posting, the thumbnail will automatically show up on pixiv.
(3) Use the Doodle feature within your group
You have the option of posting an illustration in your group with the Doodle tool.
Check out this page for more details.