Does anyone know how I can change the pictures of the paintings in Framing Frames?

[Stolas! ❤]

I've seen mods that are able to do it but I just wanted to learn how to do it myself so I can replace the pictures with my own.

[Yamaha]

First and foremost you have to extract the assets, this guide covers that topic (it's outdated but the method hasn't changed).
https://steamcommunity.com/sharedfiles/filedetails/?id=231568439

You will need:

• Bundle Modder[modworkshop.net]
  to extract the game's files
  
• Hashlist (old, should work)[bitbucket.org]
  
• Hashlist (new)[github.com]
  an additional document for Bundle Modder to work with, so the files are extracted correctly
  
• Nvidia's .dds Plugin[developer.nvidia.com]
  to be able to work with/open .dds files once those are converted from .texture to .dds
  
• Photoshop[www.photoshop.com]
  for image/texture editing

Once game files are extracted, the textures you need can be found in "units\payday2\props\shared_textures"

Anything that starts with

com_int_gallery_wall_painting

is for the artwork.