Disable image size settings if filters are being used

As explained in #5533, this disables image size settings that are technically doing nothing as soon as their respective filters are being used. It also shows a small description right below the image size label in the administration panel. Example screenshot:

![Disabled setting for image size](http://f.cl.ly/items/2x1L2c2A2M1q2b2t460z/Screen%20Shot%202014-05-21%20at%2016.29.48.png)

@jameskoster You might wanna check this for the UI/UX parts of this change.

Closes #5533 if merged in.