Well... it seems that the reason I was thinking of a restore is no longer a problem in any case. Suddenly messages from my sister are once again showing image attachments as normal...
Perhaps I should start a new thread about why entering Time Machine won't show whatever I've selected in Finder no matter what I select? Hopefully someone might have an idea about that... all I can find online is mention of some problems with Time Machine with El Capitan but they don't apply to me and I've never had a problem until now.