I have made a tutorial about creating a weapon that discusses making an icon for an item. (Step 1 of this link :
https://titanquestfans.net/index.php?topic=886.0) It uses an existing texture but you can make your own from scratch, import images online or use BitmapCreator to create new icons from existing icons and custom lightning (instructions here :
https://titanquestfans.net/index.php?topic=776.0)
An important thing to keep in mind is that the dimensions for the inventory are in increments of 32 pixels (1 square). For example, helmets are sized 64px by 64px, and most torsos 64px by 96px. If you have a number off of that, it will round up to the next increment. For example, if you have a texture 37px by 78px, it will be treated as 2 squares by 3 squares.