Katrin, we use some capabilities of the wiki type attribute, including showing images in this workshop - end of lab 5: https://jazz.net/library/article/1093

This relies on an internal mechanism that automatically publishes work item attachments, including image files it recognizes. I am not aware that you can just upload a picture and show it. If you add a link to a picture, I would also expect it to only show the link and not the image.

For more information, see https://jazz.net/forum/questions/86833/web-client-does-not-display-images-instead-displays-links?page=1&focusedAnswerId=86834#86834

You can subscribe to this enhancement to be kept up to date on the status of the functionality to add support for img tags and other xhtml to a wiki attribute:
https://jazz.net/jazz/web/projects/Rational%20Team%20Concert#action=com.ibm.team.workitem.viewWorkItem&id=228452