I'll answer #2:
In short, you find an image on the internet that you want to be your background, download it, and set it as your wallpaper. Beyond this, it's a least partially an issue of your personal preferences.
Find the image you want on the internet, right click on it, and select "Save image as" or something to that effect. A dialogue box should pop up prompting where you want to save it. By default, it's probably either your desktop or your home directory. I can't remember for sure. I personally recommend creating a separate directory for any backgrounds you want. To do that, click the "browse other folders" tab, and click "create." It will prompt you for a name. Then just click save, and the image will download to that directory. Just make sure you know where to find that directory.
From here, you just need to set this as your background. The simplest way is to right click, go to "Settings: Desktop Settings" and somewhere in there, you'll be able to select an image for your background. I'm not at my Vector computer right now, so I can't remember the details. If you have more questions, don't hesitate to ask. If you are familiar with doing this with Windows or MacOS, the process is nearly identical.
I don't have time to address #3 now, but I should be able to answer at least some of it. Good luck.
Thanks. I was going to save them in the same place as where the currently installed Vector wallpapers are. Do you know where that is?
Temp files created by the system are usually wiped on shutdown. For Firefox browser cache, cookies etc that's an easy one: Edit, Preferences, Privacy, Private Data, Clear Now/Settings.
Yes, I know how to clear them using the browser settings, but I was referring to the system-wide files.
Is running "sudo apt-get clean" effective? I tried it and it said command not found.