callbackBeforeUpdate 2 images

I have a script that converts the user uploaded image to base64 using a temporary table.
I usually
callbackBeforeUpload
to convert data to base4
and
callbackBeforeUpdate to change state parameters
Everything works fine with a single image.

Now I need to change 2 images.

The question is how do I identify which of the two images called callbackBeforeUpload?