Well you have two options here. Either upload the images and attach them later to the event whenever it's created or create the event before you go to the create form.
So for the first option you can store the path to the event in the session for example and then later reference that session to attach the images. However when images are still in session and the user click away the browser you probably have some images on your server that have never been attached to the event. You can for example store the images in a different location when creating and still save the reference in the session. You can then have a cronjob running everyday to clear out the images in that directory. Once the user saves the form you move the images to the correct directory and they will never be deleted.
For the other solution you kinda have the same problem. You can create the event beforehand and show that to the user, this way you already have an id. However this also means that if the user never finishes creating the event you have an empty event hanging around. This makes it easier for the image upload part, but you're still stuck with dangling events. You can for example set a flag on if it's been created by the user or by the system so you know which one you can delete, including the connected images.
Personally I would go for option one since you need to find a solution for the images and not for the event itself