Where can I find keyboard keys and mouse buttons textures?

That would be 0f000001.pack. It loads only at startup, not on rooms changes, so if you modify this file, you will need to restart your game before you see any changes.

Well this is a quick guide to navigate the ImagePack folder

00000009
00000071 Enemies

0d100000
0e333100 Cutscenes textures packs

01000000
01000021 Players

07000000 Holds important stuff, like specular (of water), explosion, fog, the film grain sfxs and most of the HUD

02000000
02000055 In-game weapons

22000000
22000097 Inventory items (no weapons)

23000011
29000001 End's Chapter Screens

30000001
39000001 Misc Menus Screens including Mercenaries result screens
and inventory UI

3f0001a1
3f0029a1 Files and playing manual mixed together

3f0051a1
3f0055a1 Separate Ways Begin Chapter Screen

43000000
4300009f Examine Items

44000002
4400061f Stages

The games order the packs in this format 001-009 00a-00f then 010-019 01a-01f
Also if you see packs of 6 files repeating the same textures, they are mostly language dependent, for example files ending in 01 corresponds to english and 00 corresponds to japanese

There you go, I know this list is by no means complete since the number of files are overwhelmingly large