Martyn,
... I'd would like to know if it is possible to find the total
storage requirement of the Yahoo group - at present all I can see is
the size allocated to files - not images or posts.
Posts (the message texts) are not counted against one's storage allocation, on either Yahoo Groups or Groups.io.
For Photos go to the Photos section then switch to Albums. It will show your usage at the top (below the tabs, but above the first row of albums).
For Message Attachments I'm not sure. My understanding is that they count towards the same 100 GB limit as Photos, but I don't see a place to find out their size, nor do I know if they are included in the size shown for Photos.
Shal
Shal Farley