All surface textures that are based on images (vs procedural ones for Cineware/Redshift) are in the loaded libraries.
That said, to extract the image requires extracting the libraries so that the folders that you see within Archicad will be actual folders on your computer that you can browse to locate and copy-then-modify the image. (The modified image can be placed in your embedded library for the project and used from there unless it is an image that you expect to use in many projects in the future.)
How you access the image file (most are indeed jpg's) depends on if the surface texture image is associated with the Basic Engine (what you see in the 3D window and certain renders) - or with the Cineware/Redshift engine (which has multiple images serving different functions).
For the Basic Engine, make a note of the texture file name - and click the folder icon to the right. While this is intended to browse for a replacement texture, it does let you browse the library folder hierarchy to see just where this image file is located.
If the texture file is part of a Cineware/Redshift surface/shader, then the process is different. Here, you find the image and then in the detail panel, click the 3 dots (...) to the right of the file name to find out where it comes from (or to replace it - which isn't your goal, I know - but it is the actual purpose of the 3 dots). My screenshot is showing the Diffusion image... but the Color image is the main texture that you see. (If making a revision to an existing Cineware surface, you would want to grab any and all images used and modify all of them in Photoshop/whatever to create your modified surface.)
Finally... how do you extract the contents of a library LCF file onto your disk in order to see all of the content in File Explorer / Finder - the jpg images in this case? The command File > Libraries and Objects > Extract a Container... lets you browse for the desired LCF file (typically inside your Archicad application folder for standard libraries - but could be a company library on a server somewhere) and give a destination.
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