Just wondering if there's a way to create a "stop action" if I am requesting a recipient of my form to upload a photo and they forget to do so before submitting an order (i.e. - "Order cannot be completed until you upload a photo.").
Also, if there is a way to do this, what if I offer a "non-photo" option as well as a "photo" option on the same form? (A customer chooses a product where no photo is required so they can submit the form without uploading a photo, yet if they choose the "photo" option and forget to upload a photo, how can I stop the form from being submitted so they can complete that action?)
You can mark the attachment field as required, to make sure that a photo is uploaded during submit.
If you want to make it conditional (to show/hide the attachment field), you can try to use Skip Logic, please see here for more info: