If the tenant gives you permission to enter the property and take a few photo's, then call a locksmith to open the door. This is very simple, many people lose their keys or lock themselves out of their apartment every day and its not difficult for a locksmith to do this.
But anyway, if you already have photo's of the interior prior to the tenant moving in, they should be good enough. The photos just show overall layout/size, etc.
I'm more interested to know when does the tenancy agreement end? You are planning to sell it with a tenant, which is fine as long as is advertised as such and new owners must be made aware of and continue to honour existing agreement. Or you are planning to terminate the agreement early and pay some compensation to tenant?