You can clear the cache by selecting Clear Image Cache under Sync Settings in the app settings. You may also want to consider adjusting the following settings to reduce the amount of storage space needed:
To disable pre-downloading images for offline reading, change the Unread Items and Starred Items settings to off under Sync Settings->Download Images in the app settings. Images will still displayed when online but will not be downloaded before they are needed.
You may also consider reducing the Max Sync Items and Keep Read Items settings under Sync Settings as well. This will reduce the overall storage space required by reducing the number of articles and images that are kept on your device.