Go into your gallery to see your images. Check the box in the upper left of one or more images.
Then a window pops up where you can chose to move the image(s) to a new album or an existing album.
Unfortunately the option to move to an "existing album" is a bit of a pain because it doesn't let you chose from your existing albums, like in a dropdown menu. It requires you provide the URL of the album you want the image(s) moved to. So before you do this step, you'll need to first find the existing album and copy the URL (web address) and then paste it into this window.