You can add multiple photos to a post by clicking on "more attachments" just below where you've added your first photo.

If you run into a problem, it may be because you're trying load enormous photo files taken on your fancy phone camera   There is currently a limit on the maximum file size of attachments on a post.

See also:
https://cyclekartsgb.createaforum.com/announcements/how-to-add-images-to-a-post/