Hello. I made a morphed accessory using a supernatural ep necklace as my donor. I added my custom thumbnail in tsrw, exported it, and used S3PE to paste the casp instance number to the thumbnail. But when I tested it, the game was showing its own generated thumbnail instead of mine. I remember I read a tutorial on it a long time ago, and I think it mentioned changing the group number but I don't remember what number it was. How do I add custom thumbnails to morphed accessories using s3pe?

First, open the Documents / Electronic Arts / The Sims 3 / Thumbnails folder, and delete the CASThumbnails package.
Then open the game with your CC installed and navigate to it, so that the thumbnail generates.

Quit the game and open the CASThumbnails package - click through the resources until you find your CC. That resource will have the correct Group and ID that your custom thumbnail should have - the group will vary depending on the expansion and preset number.
Make sure to delete CASThumbnails again before testing.

While the new thumbnail is open is s3pe, you also must change the "Tag". What I do, is not replace the image in TSRW, but the entire process in s3pe, converting the icon and replacing that image.

1, compress the package as usual.
2, right click CASP, then select "Copy ResourceKey".
3, double click the icon and paste the resource key.
4, at the top of that box, click the down arrow next to "Type". Yours may say it's still a CASP? Scroll thru the list and select "THUM 0x626F60CE". There are 3 sizes. the others are used for objects.
5, change ONLY the last digit in the group number to "1". If the CC is cloned from base game, an EP or SP, the group number must remain the same as the CASP, except the last digit. If you have more than 1 preset, change the last digit of the group to "2", "3" etc.
6, Last, right click the new THUM, select "replace"... find your image, and click on it. THUM images MUST be 256x256!

NOTE: Hair is different! Without accys is "0x00000000", and with accys is "0x00000001". I usually convert CC to base game, unless it uses some special script, related to an EP.

This worked! Thank you for the detailed steps and note.

Thank you, this method worked for me! I didn't delete cas thumbnails last time, but I'll remember to do that in the future