When selecting multiple items for example on Android Google Photos the checkmark is placed on the top left corner but on iOS Photos the checkmark is placed on the bottom right corner. What do you think is the optimal placement for the checkmark?
I think the checkmark position is negligible, as long as it is within that item and it in a consistent position. (Note: that would mean the item dimentions must also be consistent to form an aligned grid.)
However! There might be some difference depending on how you want user to perceive the checkmark:
- If you want to indicate that the checkmark is in fact on/off state button, place it in right bottom corner
- If you want to indicate that the checkmark is just notifying of the item state, then the position other than that one is suggested
The reason for that, is that the finger on mobile device comes from bottom right for majority of the users (with exception or left handed people etc.). Therefore, people will tap the item from that direction and if the checkmark appears there, the natural easiness of tapping it would suggest that it is a button.
Placing checkmarks on the left of the image is going to be better for right handed users as their thumb is less likely to obscure them but this is also bad for lefties.
Placing checkmarks in the top of an image is great for images at the top of the screen where the item is less likely to be obscured by the user, but, at the bottom of the screen, the opposite is true - take your pick!