Add the images to the same slot.
Just drag multiple images from the images node into the same slot in the tree. It should be straightforward.
In Animate mode, key the images frame by frame.
This is kinda tedious currently. Hotkeys help a lot though.
Instead of manually clicking on the frame number with the mouse.
R
for "next frame". also F
for "previous frame"
Instead of clicking the key icon on the slot with the mouse.
K
for key change.
With this, you can keep your mouse on the tree, and just use hotkeys to key and move the frame forward. That makes task much easier than doing everything with the mouse.