Images on external or your own servers are not necessarily accessible from the Matrix server or they might disappear in the future.
So the images might be visible on your PC, e.g. because your are logged on to your confluence but not for our server or your colleagues.
What do do in this case:
- download the image to your computer
- remove the link in Matrix
- import it into the text field (either browsing for it or using drag and drop)
From 1.11 on, Matrix tries to automatically import external images - if this fails an error message is displayed in the client.