Synth
Requirements:
Tool to export textures from the game files. (e.g. https://tom-crowley.co.uk/downloads/?id=15, this dumps whole .ipak files, there are other tools but i never used them so no idea how they work).
Photoshop with nvidia or intel dds plugin (google it, again, might work with free image editing software and different plugins but this is what i use).
kokole's iwi x dds fast converter to convert your modified textures back to bo2's .iwi format.
Basically:
Dump .ipak(s) to get textures in dds format
Open texture in photoshop and modify it to your liking
Export modified texture as dds
Convert modified texture from dds to iwi (drop it onto the iwi x dds converter exe)
Put .iwi file into t6r/data/images (create that folder structure if it doesn't exist in your bo2 folder)