The first thing I to try is clearing the app’s image cache.
To do this, Open TweetCaster --> click 3-dotted icon in upper right corner --> click "Settings" --> "System" --> "Clear Cache”.
You may also want to clear the main cache by going to your phone's main screen, tapping menu --> settings --> applications -- > Manage Applications --> TweetCaster and "clear cache".
The underlying cause of this is usually low memory on the device. If there are any unused files you can delete from your phone to free up some space, that will likely help prevent this from occurring in the future.