1) Get an image that you like.

2) Open it in Photoshop/Gimp/Paint. I personally recommend using Photoshop and change the pic size to a power of 2 (Ex. 1024x1024, 512x512, etc.)

3) Save the image as a DDS/TGA in root/raw/images. Works either way. Just use TGA for your case and just leave the TGA settings as it is.

4) Open up Asset Manager and follow the settings that are on the image that I showed you. (You must remember what you named it)

5) Convert it by pressing F10 and it should say 'Conversion Done.'

6) Copy the converted file in root/raw/images and paste it in your root/mods/YOURMODNAME/images folder.

7) For the material to work on the gun, you go into root/raw/xmodel and find the viewmodel of your gun. Since you are using the Beretta, find viewmodel_beretta. Make a copy of that file and rename it something like viewmodel_beretta_new_tex. Open the file with Notepad++(Download it if you don't have it) and find 'mtl_weapon_beretta'. Replace that with what you named your material in Step 4. 

8) Find your beretta file in root/mods/YOURMODNAME/weapons/sp, which supposedly should be named beretta. Make a copy of this as well and rename it beretta_new_tex(The reason for this is to keep the original beretta gun instead of replacing it). Open it up with Notepad++ and find 'viewmodel_beretta'. Replace that with what you named your viewmodel file in root/raw/xmodel. Save/close.

9) Assuming that you ported this gun over from COD 4, insert this line 'weapon,sp\beretta_new_tex' in your mod.csv(It is the box beside the IWD list in Mod Builder) Remember to include this weapon in _zombiemods_weapons and dlc3_code(Copy this from the raw/maps folder and paste it in YOURMODNAME/maps folder if you haven't already)

9) Check the image in the IWD List in Mod Builder. Compile and Build Mod.