A wallet pass has a small, fixed set of places an image can go, and each has a job. On Apple Wallet the slots are named files inside the PKPass bundle, and which of them the wallet draws depends on the pass style declared in pass.json.
The icon is required on every pass. It is what the wallet shows in notifications and on the lock screen, and it is small, so it should be a mark rather than a wordmark. The logo sits top left on the pass face, next to the logo text, and is the brand's main appearance. The strip image runs behind the primary field on store cards, coupons and event tickets, and is where most loyalty passes carry their personality. The thumbnail is a small portrait next to the primary field on generic passes, suited to a member photo or a product. The background image is reserved for event tickets, where it is blurred behind the pass, and the footer for boarding passes, above the barcode. Newer poster-style passes replace several of these with a single full-bleed artwork.
Each image is supplied at one, two and three times its base size, so it stays crisp on every screen, and images can be swapped per language through localization. The bundle is signed as a whole, so changing an image means reissuing the pass; a program that wants seasonal artwork should plan for a reissue cycle rather than a live swap.
Google Wallet has fewer slots. A program logo appears at the top of every pass, the hero image runs full width beneath the fields, and image modules can add more further down. Because the images are referenced by URL from the pass object rather than packed into a file, they can be changed without reissuing.
The practical rule across both wallets: text lives in fields, not in images. A balance baked into a strip image cannot update, cannot be localized and disappears on a watch.