Welcome to Atlassian Community! It's nice to have you join us.
One thing that could happen is if the images in the page were copied and pasted from another location, rather than attached physically to the page. So, when you export the space, they won't come along.
However, you mentioned the space is a Documentation Theme space, but the Documentation Theme has been discontinued since Confluence 6.0. Only Confluence versions later than 6.13 are eligible for import in Cloud:
Space import will not work if you are importing from Confluence Server version 6.13 or earlier.
To avoid any issues, we recommend using the Confluence Cloud Migration Assistant for all server to cloud imports. The migration assistant can be used with Confluence Server versions 5.10 and later.
Are you able to confirm that your Confluence site is a version later than 6.13, and the images that are missing were directly attached to the page, and not copied and pasted in?
Hello Community! Quick disclaimer: We are running a contest on Community (The Atlympics!) from July 23rd - August 8th of 2021. If you are interested in participating in this contest (prizes! ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events