I recently uploaded a series of four images, and then created a pool for them ("Mia Fey's Duty"). However, I quickly discovered that I couldn't, no matter how hard I tried, figure out how to put images IN a pool - not since the site migration, anyway. Looking it up yielded no answer.
Someone's added the 2nd, 3rd and 4th picture to the pool, but left out the first. Anyone can explain to me how to do this?

enter to the pool, >import > copy the tags

What do you mean by "tags"? The post's ID?

Copy the pool's ID number (last four digits of the url) and tag the pictures with pool:XXXX.

Thank you!

Unfortunately, this method doesn't seem to work with personal pools (it works for private and public). You can use the pool import to find a specific post by searching the post id, like this:
id:1234
and it should show up. You can also use the import search box for any normal search, like an artist's name, and then you can pick which ones you want to add to the pool.