Deadass, just go over and download Solos_Poster_Changer, not this one if you want to make a custom poster mod. He's the one that put together the .dll to make all this possible. I know absolutely nothing about that. This is how I did it.

Files are contained in BepInEx\plugins\Raene-Dumb_Posters_1\SolosPosterChanger

Inside this folder are a bunch of .png files and a .dll (do not touch the latter)

There are 4 sets of posters, 4 connected to the clipboard (manual1.png, manual2v2.png, etc), changing every page of the manual that spawns with the ship. 1 poster pertaining to tips (TipsPoster2.png), and a collection of posters scattered around the ship (posters.png).

1. Open the image you want to change in Photoshop. This matters because you need to fit the scale of the posters, too big or small and there will be cut offs)

2. For the manual, beware of cutoffs at the top). For the posters collection, just scale and transform whatever picture you want to add over your desired photo. Not too difficult if you just toggle layers and adjust accordingly. For the Tips poster, this is by far the easiest, basically just replace the photo.

3. Export as a .png and replace the photo in the folder. The filename has to match in over to overwrite.

4. Once everything has been replaced, in order to share these posters with friends over thunderstore, you kind of need to reupload this mod like I did. Go into the manifest.json in the folder that contains plugins_SolosPosterChanger file and basically change everything except the dependencies. This is important otherwise you won't be able to upload since the system will assume you're trying to overwrite the original mod. And also it's just full on ripping off.

5. Zip all the files (manifest.json, readme.md, icon.png, plugins), then head over to Thunderstore and go to the upload tab of LethalCompany in the top left. Make sure before uploading you choose the modpack category.

All credit goes to the original mod author.