This is an extract from the reply I received from Yahoo Groups, I thought I'd share it.
I understand you are receiving a "HTTP 500 error" when attempting to
access photos within your Yahoo! Groups. I apologize for any frustration
or inconvenience this has caused.
We are aware of the issue and have identified that any user with a
Yahoo! ID greater than 32 characters is unable to successfully view
photos within Yahoo! Groups. Our Engineers are working on a resolution;
unfortunately the changes we need to make are fairly significant so we
can't quickly resolve the issue. However, we are treating this as a
priority and hope to have a fix in place in the next few weeks.
In the meantime, you will be able to view the photos if you join the
group with an alternative Yahoo! ID that is shorter than 32 characters.
At the moment I don't know if I will go with this new identity bit, as I don't want the hassle of chaning my e-mail (if that is what it involves) I may just wait a little longer and see if they solve it.
Thank you to everyone who contacted me with suggestions.