Hi Charlie,
Good question! We do indeed strip HTML out of text inputs as a security measure–otherwise, malicious code could be inserted into your site unbeknownst to you.
If you’re looking for an image upload option, I would suggest using Advanced Custom Fields and assigning a custom image upload field for the Events post type. From there, you can use a template override (see our Themer’s Guide for more info) to insert the image into the event template where you would like it to display on the front end.
Will that work for you? Please let me know. 🙂
Cheers,
Geoff