It's really easy but a bit time-consuming. Replacing the default pictures with ones you like better is the quickest way, but if you want to add more then you need to go into the folder you want to add to and edit the imagemeta.js file with a program like Notepad++. You'll see how entries are formatted in the file itself; just copy/paste old ones and change the number to the next highest and repeat that for however many images you want to add. Then just move your new pictures into that folder and make sure they're named to correspond to the numbers you entered in the .js file.
So you'd copy one of the entries that looks like this
7: {
title: "Black Claw",
artist: "Noxypia",
url: "
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",
license: "CC-BY-NC-ND 3.0",
},
and paste it below this entry, change the number to 8, then name your picture 8 and put it in the folder.