How to fire a Rule when an image is removed from Image field with unlimited values?
To my knowledge there is no Rules Event like the one you are looking for. However, as illustrated in my answer to the question you linked to, you can use an approach like so:


iterate over all field values of your field of the unchanged node.
within each iteration step, check if the list of items in the changed node does NOT contain the field value being processed in the current iteration step ... If it is NOT contained in it anymore, it means  the values is removed.


Note: processing image fields in Rules is a bit special. You may have to use a token like [node:field-pictures:file:url] and [list-item:file:url]. And if they appear to be empty, then retry by adding a Rules Condition like "content is of type = article"