https://sims4studio.com/thread/1019/make-custom-catalog-thumbnail?page=1

Introduction

This tutorial will show you how to create a custom catalog thumbnail using Sims 4 Studio. The same steps are used for CAS items and object items. This tutorial assumes you know how to create a new mesh or recolor using Sims 4 Studio and will not go over those steps. Also, be sure to check out Studio's catalog thumbnail overlay feature. It's a huge time saver and the tutorial showing how to use it is HERE.

What you will need

Sims 4 Studio 2.4.0.7 (Love) or above

Instructions

Create your new recolor or mesh. When you're finished click Save. If you want to create your own unique thumbnail make it as a 104 x 148 .png and import that into Studio. If you want a game generated thumbnail to place your icon on continue with the following steps:

  1. Put the .package in your game Mods folder as usual. Close Studio.

  1. Check your item in the game. You should see the normal game generated catalog thumbnail. Open the swatch thumbnail menu and run your mouse over it so that each of your swatches appears on the model shown in the catalog thumbnail picture that the arrow is pointing to below. If you do not see each of your swatches shown on the model in this picture Studio will not be able to generate an EA style thumbnail for that swatch.

  1. If your item is a CAS item that you've designated for use by both genders make sure to perform step 2 for both male and female Sims.

  1. Close the game and reopen Studio. Open your .package using the Recent Projects list or the My Projects button. Either way will work. Export the thumbnail that Studio has generated and open it in your 2D editor. Add your custom icon to the thumbnail and save it as a .png.

  1. Import your new thumbnail back into Studio to overwrite the original one that was generated by the game. If your item is a CAS item enabled for both genders repeat the process for the other catalog thumbnail. Make sure to create a custom thumbnail for each swatch in the .package. Save the .package and place it in your Mods folder so that it overwrites the first one you put there. Start the game. There is no need to empty the cache to see the new thumbnail you've made as Studio has removed the old thumbnail from the cache already. This will allow the game to use your new one.