Thank you, it worked!
For others, here is my working code:
(Don't forget to put the speed of the animation to 0, so the animation doesn't play to the last frame, like I did at first)
+ FileChooser: On changed
-> Sprite: Add animation frame to "Animation" at -1
-> Sprite: Set animation frame to Sprite.AnimationFrameCount-1
-> Sprite: Load image from FileChooser.FileURLAt(0) (Keep current size, cross-origin anonymous)
----+ Sprite: Pick instance with UID 2
-----> Sprite: Set animation frame to 0
-----> Sprite: Set size to (250, 250)
----+ Sprite: Pick instance with UID 3
-----> Sprite: Set animation frame to 1
-----> Sprite: Set size to (250, 250)