We gave Play Store version 5.9 the teardown treatment a few days ago, but it turns out we missed something. The latest release no longer automatically downloads app screenshots when you're not connected to Wi-Fi or LTE.
When on 3G or HSPA+, you will see vertical gray placeholders with download arrows on them in place of screenshots. Tapping them initiates a manual download.
When we tested things out, the arrows didn't go away after returning to LTE. But they did begin to pop up as usual after force closing the app.
This feature reduces the burden your device puts on slower networks. It might also cut back on how much cellular data some users burn through, though by automatically loading on LTE, this isn't the primary purpose.
If you don't have 5.9 yet, you know where you can find it.