Okay, now you've added something! But it doesn't have a preview icon yet - you'll have to add this manually. For this purpose, you'll have to edit an image.
For a guide how to ensure image transparency, see here.
Cyberpunk uses xbm as format for its textures. These textures are then mapped (divided into slices) by inkatlas files. The individual slots can then be used by the game for pretty much everything from UI elements to phone call icons — and image previews.
If you want to use the in-game previews, you can find them under base\gameplay\gui\common\icons\items
Setting up the files
First of all, download the template archive (kindly provided by Apart). This includes the following files:
Template
Size of slot image
5_outfits
160x320 (x5)
5_weapons
360x120 (x5)
40_items_inkatlas_template
160x160 (x40)
This guide assumes that you have a bunch of ready-made icons lying around - if you don't and have no idea where to start, you can check this link, section "Making the icon".
Select 40_items_inkatlas_template and copy the .inkatlas and the .xbm to a folder of your choice inside your Wolvenkit project( this guide will be using tutorial\ops).
I'm renaming both of them to preview_icons so that future me won't be confused about this: