1. Upload the image, once done create a row and return it's id
2. back in the browser store the id in a hidden input
3. when the user submits the form send the image id in the hidden input
4. back in the form post method find the image by id and attach to the specific form record

I'd rather upload the images when the form is submitted instead of uploading them when they're selected.

then why not do that ??

Please reread my question.

reread my solution, i've been using it for nearly 8 years now. and it works. if your problem is having trash rows on the db, consider running a garbage collection script and clean them