To clarify isset and == are two separate check and must be treated as such. isset is checking for the existence of the variable while == is looking for a specific value.
I am trying to upload an image.
Preview the image.
Then go back into edit mode and choose to keep, delete or replace the image.
Then preview it again, before submitting it to a database.
One of the challenges I run into is when I go back and forth between between edit and preview with a new browser window, parts of the script try to repeat itself.