Update: I made a GUI for it (Windows only)

Just out of interest, I wanted to know if there is an easier way to create wallpaper for the phone. So I created these tools.
Maybe you'll find it helpful too.
Only for people who don't want to install Unreal Engine etc.

I tried to make it so that you don't have to install anything. The required Python libs are already included.
Tools used:
https://github.com/matyalatte/UE4-DDS-Tools
https://github.com/panzi/u4pak

Usage:

Use the GUI version or if you want to use the batch version:

If you want to create a non-animated phone wallpaper, then go to folder
_single

1. In the png folder replace T_PhoneScreenWithAlpha.png with your own png
2. Doubleclick on start.bat
3. Rename created yourmodname_phone.pak and put it in your PAYDAY3 Mods folder

================================================================================================

If you want to create an animated phone wallpaper, then go to folder
_multi

1. Put your pngs into the _png folder (120 PNG files required, the minimum doesn't matter as it will be created automatically for you)
2. Doubleclick on start.bat
3. Rename created yourmodname_phone.pak and put it in your PAYDAY3 Mods folder

If the animation is too fast, then create more frames for each frame:
before: Frame1, Frame2, Frame3 etc
after: Frame1, Frame1, Frame1, Frame2, Frame2, Frame2, Frame3, Frame3, Frame3 etc
(you can do it easily by selecting all the pngs, copying and pasting. now each frame has another one.)

If you need more help, look in the _png_helper folder in _single and _multi
There are some batch files that can help you

Note: if your wallpaper doesn't work, make sure you use -fileopenlog and the mod "zModLoader_P.pak" is in the mods folder.

Animated Function and Advanced User Tutorial by Shalashaska
https://modworkshop.net/mod/44827?tab=instructions
Thank you!