These tools allow you to remove device-specific and unnecessary images from your iDevice.
As an example, iPad images are not needed on iPhone and iPod, and can be safely removed.
Similarly, 4 inch images can be removed on every iOS device, except those with a 4" display (such as the iPhone 5 or 5s).
Images with a higher resolution than the display of your device can also be removed without trouble.
Caution is advised, instead, if you're attempting to remove images with a lower resolution, as I received multiple reports of apps breaking when this operation is attempted.
Because of this, a test mode run is strongly recommended.
iTunesArtwork files are only needed for ad-hoc app distribution, display on the AppStore and iTunes library. They serve no purpose on the device. Every app you install has one, and it can be safely removed.
As a result, however, iTunes won't show artworks for apps in the library. If you want to be able to view them, please "transfer your purchases" on iTunes before removing iTunesArtwork files.
The images removal tools may break some applications. If that's the case, and you enabled test mode beforehand, you can tap on restore backup files in order to undo the changes made by iCleaner. If you did not, you can still reinstall the broken apps. Make sure to exclude them via the excluded apps menu.
iCleaner scans for images only in user app bundle folders. It does not scan the whole disk as a security measure.
Removing images (and languages) will also affect .ipa files transferred from the device to iTunes. This is especially important if you synchronize your apps to both your iPhone and iPad, as deleting iPad images on your iPhone will break the app on your iPad.
Same story if you have both a 3.5" and a 4" device. To avoid this, it's mandatory that you "transfer your purchases" on iTunes before you use iCleaner to remove images and/or languages.