Good to hear you found a way that works for you. Alternatives that come to my mind would be
Putting everything into a single collective and using Pages and subpages for structuring the different aspects
Allowing Groups in circles (admin setting) and then adding everyone to one group and linking that group to every collectives circle.
We just released a new version that has some fixes for displaying images. Not sure if we are covering the issue you are seeing.
To narrow it down could you answer a few questions?
Which version of nextcloud are you using?
Which version of the collectives app are you using?
Can you see the images when opening the .md file in the files app?
Can you see the images when opening the corresponding page without editing it?
Can you see the images when editing the page?
Could you upload the markdown file in question?
When the images do not display you should see a failing request in the developer tools of your browser. (Usually you can open those by pressing ctrl, shift and “i” or “j” at the same time. Click on the network tab and then open the page in question. Look for a red line that has a response code of ‘404’ and looks like it tries to load the image. Maybe you can attach a screenshot of the network requests or the link that it failed to load.