How do i delete an image that has been uploaded to a form from someone?

You will need to delete whole submission. It is not possible to delete only uploaded file. Inside of this guide you can find how to delete  form submission data:

https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data 

Please note: Delete submissions are permanently deleted and can no longer be restored. 

I would suggest you to first download your files to your computer and then delete them. 

How to Download Form Submissions as Excel/CSV/PDF?

Hope thsi will help. Let us know if you need further assistance.