https://www.patreon.com/posts/how-to-add-icons-42171247

This tutorial will not cover creating icons/images in a photo editing program, only adding a custom one to your files.

Hopefully you already have a mod going, and the files you want to add the image to done. To add an icon, you need to make sure you are adding the right resolution (or as I call it, size, lol) and transparency. If your file already has an icon referenced, please use that as a base. If your file does not, open similar EA files to see what icons they are using and use those as a base. That’s probably confusing, but here is an example.

Step 1: I want to add an icon to my interaction called Plead for Powers. My interaction originally did not come with an icon, so I had to look at similar interactions that used icons as a base. This also allowed me to copy the format and add it to mine. The file I looked at was E882D22F!00000026!000000000003AB20.knitting_DispelCurse.InteractionTuning.xml which is the Reverse the Sweater Curse interaction. First, I copy/pasted the pie_menu_icon code to my file, then I copied the instance of the icon.

Step 2: Go to Tools > Game File Cruiser.

Step 3: Once it opens paste the instance you copied in the Instance space, then filter to DST or just hit All. The icon that was referenced should now appear. Click on it then click “Add to current package”.

Step 4: You now have a base icon. Go ahead and give it a new instance (seen in Option 2 of this tutorial if you are unfamiliar with the process). Now note the resolution/size of your base icon, which for me is 30x30. So, when I go and make my custom icon I have to keep in mind how large it is and try to stay within that range to avoid visual issues. Sometimes smaller works, and rarely does bigger work. Generally, it's recommended to make the images/icons in Sims 4 with a power of 2, but sometimes you can use EA sizes without issue. If you run into issues though, it's best to try a power of 2 next. For me, I actually had an icon in mind already and instead used the Vampire Powers icon, which is 40x40.

Step 5: Go ahead and make your icon, save with transparency (so .PNG only, not JPEG, etc.) then when done import it to replace the original.

Step 6: Now copy the new instance of your icon and go back to the file whose reference you want to replace and paste over the original. If you have a SimData don’t forget to replace the instance there as well. Ignore the p="InGame\UI\Icons\PieMenu\Skills\skill_knitting_piemenu.png" and similar in your files because they don’t require replacing. They are leftover bits from EA’s files that we don’t need to acknowledge.

Save, delete your localthumbcache and test in-game to make sure it works. If for some reason it appears as a gray llama icon instead, it probably means you are using the wrong size, didn’t clear your localthumbcache, or another problem exists. Try resizing the icon after looking at more references, and make sure all parts of the key are correct (type, group, and instance).

(Note that regular tuning files use a different Type (2f7d0004) for icons than seen in the SimData files (00B2D882), no idea why but don’t change those.)