Image field is always an array, it's just that page output formatting does change it so in template files you get an object if the image field is set to max 1 item.
So if output formatting is off it's always treated as an array, disregarding the field settings. I think in case of a bootstrap there's no output formatting on I think.
That's good to know. Forgot about the fact that image fields can have multiple items, so by default it would make sense to be an array. Thanks!