In your examples, there is no significant difference in meaning. The definitions I'm providing are just to give you a sense of how they might be different.
(1)to collect snails from my garden (the snails came from the garden)
to collect snails in my garden (I collected the snails in the garden)
(2)Your stomach would turn at the idea of frying potatoes with animal fat. (I used animal fat to fry the potatoes)
Your stomach would turn at the idea of frying potatoes in animal fat. (The potatoes were fried in a pan of animal fat)
I wouldn't split hairs over the slight difference in meaning; it's not worth the time.