By default you can add a banner and logo to your event homepage under Settings > Event homepage:
If you wish to add an image to the Short description or Additional info fields, you'll need to host it somewhere publicly online first and then right-click on the image file to copy its direct URL. You can then embed it using Markdown.
The format is:
![alt text](http://example.com/image.jpg "optional title")
So…
![Cute kittens](http://placekitten.com/500/500 "So fluffy")
Becomes:
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