You must be a group admin to post to your group.
After logging in, go to Admin page, click “+ New” at the top, and select “Post”. Or use the ‘Posts’ menu from the side bar.
You’ll see the new post creator similar to pictured. Be sure to give it a title (circled). For posts to stay at the top, eg the about group post, edit visibility from right pane and enable “make post sticky” (circled)
To insert pictures into your post, use the ‘add media’ button
It is vital to select your group category from the category box on right hand side, and deselect the default ‘uncategorised’.
Once finished, click the blue ‘Publish’ or ‘Update’ button under Publish settings.