Before you do anything, you need to locate where your Photobooth Posters are stored. They're typically found in:

My Games\XCOM2 War of the Chosen\XComGame\Photobooth\Campaign_X\UserPhotos
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You can also view your photobooth gallery in-game and click the 'Open' button to have the game instantly open your poster folder.

Note that posters are separated by campaign - posters made in one campaign won't appear in subsequent campaigns, you'll have to generate a new set of posters to be replaced.

You won't need to re-edit all of those posters manually, though. Copy/paste your edited posters to an external folder, then in a new campaign generate the same amount of posters as you have meme edits, then just copy/paste the meme edits into the new campaign's photobooth folder.
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Re-launch the game, and all of your edits should be back.
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Since posters are stored as regular .png files, this means they can be easily opened and edited with most image editing software.

Using an image editing program of your choice, open a poster from your Photobooth folder.
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With the poster open, you can effectively do whatever you want to it. Drawing on it with the pencil tool, adding wacky filters and effects to it, replacing the poster with a different image entirely, etcetra.

As long as the poster retains its' original dimensions and its' format of .png, all of your edits will show up in-game.
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Posters always generate as "Poster_XXX" starting count at 000 and iterating up from there, which would imply you could potentially have hundreds of meme edit posters in a single campaign. I've never tested this though, so i don't know. Try it yourself if you really feel like it.

The only method to generate new posters the game will actually read is to open the photobooth and take some pictures as the game intends you to do, you can't just drop a bunch of posters into your photobooth folder even if they're following the correct naming convention, the game won't read them.
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