Not strict at all. A super vague picture in your mind is plenty. Mostly, you’re just trying to make it so that the radical is connected to SOMETHING that’s a real Hanzi character, so that you pick up a rough sense of where that radical can show up within a character, and later, when you’re learned more radicals, perhaps you might start remembering more characters that contain it.
The only reason those cards exist is because if they don’t, it becomes really easy for radicals to become super abstract concepts that aren’t actually attached to anything. Having to remember what each radical can be used for—at least every once in a while—will make it more likely that you can remember the radical and its mnemonic when you encounter a new Hanzi character that uses that radical.