Suggestion 1: the upper-left hand corner of each icon is what should be aligned to the grid. (I am aware that this may cause icons of differing sizes to overlap when aligned, but I'm not entirely certain "stretch icon" is easy enough to do - or popularly used enough - to compromise the "keep aligned" functionality.
Suggestion 2: One could also dim out the "keep aligned" option if any icons are stretched, given how difficult it would be to actually align/keep aligned icons of varied sizes.
Suggestion 1: the upper-left hand corner of each icon is what should be aligned to the grid. (I am aware that this may cause icons of differing sizes to overlap when aligned, but I'm not entirely certain "stretch icon" is easy enough to do - or popularly used enough - to compromise the "keep aligned" functionality.
Suggestion 2: One could also dim out the "keep aligned" option if any icons are stretched, given how difficult it would be to actually align/keep aligned icons of varied sizes.