1. POTD was cached in `~/.cache/`. But lock screen seems not a process of current user, so it won't find the cache in user's home dir. So I changed cache location to `/tmp` and it starts to work.
2. Flickr and APOD(NASA) providers cannot cache pictures in `PNG` but `JPEG` works. So I simply changed the cache format to `JPEG`. Most POTD are photos. JPEG saves more spaces.
3. Added a KDED module to cache lockscreen POTD, because lockscreen program doesn't have internet access.