I agree, the selection behavior in the images grid view is currently confusing.
Here’s the somewhat complicated back story:
In GarageSale 6 we used a custom grid view component, which put us in charge of how selection and other things worked. However, it was a few thousand lines of code we had to test and maintain ourselves.
So, for GarageSale 7 we decided to go with Apple’s grid view component. This component has some serious performance issue (notice the lag when selecting a (smart) group with 1000+ items), and shows a non-standard selection behavior. Unfortunately we discovered this only after we switched to Apple’s component.
However, there’s a new version of Apple’s grid view component available, that supposedly fixes performance and selection issues. We could switch over this newer grid view component, but only if we raised to minimum system requirement for GS 7 to OS X 10.11 or later, thus locking out a number of users.
As of now, we are still undecided what to do.