Gallery images are unlocked as follows: for every 5 images you have completed, 1 image is unlocked in the gallery. However if you have completed exactly 50 levels (all levels in the game), all gallery images are unlocked. As there are 40 gallery images but only 50 levels, gallery images 10-40 all unlock on completion of the 50th level.
But because saves are shared with MONSTERS POTION! (app 3153910), a game with 100 levels, if you have played and completed more than 50 levels in that game, you do not automatically unlock all gallery images (as you have completed more than 50 levels, not exactly 50 levels). Due to the 100 levels available in that game, the 5 levels per gallery image in Water Effect only leads to unlocking images 1-20, but not 21-40. As such, your save must be completely wiped in order to make it possible to complete this game.
Saves are located at the following registry entry. The entire registry folder must be deleted.
Computer\HKEY_CURRENT_USER\Software\Mango Anime\MONSTER POTION!
Recommended course of action:
If you have played neither game.
Complete all 50 levels of Water Effect, open every gallery image to 100% the game, and then complete levels 51-100 of MONSTERS POTION! to finish that game as well.
If you have played MONSTERS POTION! but completed <50 levels
Start Water Effect immediately, completing the remaining levels and unlocking all achievements, before returning to complete MONSTERS POTION!.
If you have completed between 51 and 99 levels in MONSTERS POTION!
Complete all remaining levels in MONSTERS POTION! and unlock all achievements, then wipe your save in the registry entry listed above before playing and completing Water Effect.
If you have unlocked all achievements in MONSTERS POTION!
Wipe your save via the registry entry listed above, then play and complete Water Effect.