You can try clearing the cache for SeriesGuide. Tap Moremenu > Settings > Clear image cache. In the App Info screen that opens, go to Storage and tap Clear Cache.
Images over Wi-Fi only is enabled
If your current connection is metered and this setting is enabled, no images are downloaded. Tap Moremenu > Settings and disable the setting.
Prevent spoilers setting is enabled
If an episode is not watched and this setting is enabled, the episode image remains hidden. Tap Moremenu > Settings > Advanced. Disable the setting.