That is what I was asked repeatedly in the previous messages.
The Admin has to add the IDs manually.
You mentioned the Admin will respond manually to the user request, and even send these users an email mentioning the access granted.
Since the Admin is adding these privileges manually, there is no other way around this.
The Admin will somehow be informed about the access request.
In that request, you could include the Post ID already, so the Admin just needs to copy-paste that into the user's field.
One can do all this automatically, of course, with a Toolset Form, but that requires more custom code, where the Admin will submit a Form to approve access (for example) and the IDs will be passed to the respective User automatically.
For example, you could have a Form that updates the user, and a generic checkboxes Field populated with a View to generate a checkable list of posts to which that user can be set to have access to.
Then you would add custom code hooks to cred_save_data() so when the form is submitted, the user field (single line with the Post IDs) gets updated.
This, however, requires more custom code, which I cannot fully prepare here, I could only assist and guide along.
I could help but you would have to mainly craft the custom code - I could not prepare a fully ready to use solution for this, due to the Support Guidelines https://toolset.com/toolset-support-policy/
This is why I wanted to clarify what you required before I created the above example, which implements it with a manual add of the ID
If you require particular help for custom code that is above the Toolset Support, which seems to be the case for this project, we can also recommend https://toolset.com/contractors/
There are other solutions, such as using a Role and upgrading the user who requests access to that role once the Admin grants access.
But this will mean the entire Role (all users of that role) will have access to the particular posts/pictures.
If you are not fully comfortable with Custom Code (mainly for maintenance) I would suggest referring to a contractor, if required.
If you are comfortable with, I can help you along creating the mechanism.