Textures for Pokemon, levels, trainers, and NPCs are all in .gfpak files. You can open these in the toolbox.
Open the quick access folder. Under "Textures" is a list of textures.
You can right click and export one as .png or .dds to view and edit in an image editor then replace back.
You can right click the "Textures" folder to batch export or replace.
The replace all function will allow the user to batch swap multiple textures that match the texture name.
If you want to view all the images quick and easy, right click the file node and select "Preview Archive".
This will open up a menu to preview all the images opened automatically. You can right click and export/replace.
More detailed info on the texture editor.
Then when it's finished, you can File - Save/Save As. Alternatively you can also CTRL + S or right click it and save.