since you are trying to unlock the image you created
image img1 = ("images/brbg_busy.png") #created a new image here
you need to show that image so
if not renpy.seen_image("img01"):
show img01
hide img01
$ renpy.notify("New gallery image unlocked")
you can eve use it in the script as the image to unlock it as well i.e.
if not renpy.seen_image("img01"):
$ renpy.notify("New gallery image unlocked")
scene img01 with dissolve:
zoom 0.5