Creating Custom Thumbnails for CAS and launcher

TSR Workshop

The thumbnails must be saved as a .png!

You may need to make sure you name them something unique, not sure if it's like s3pe but probably so. So creatorname_itemname_thumb is probably a good start.

For launcher thumbnails you need to have a 64x64 thumbnail.

For CAS it's 256x256

Open TSR workshop, and your project and get to the project tab:

Click the + beside extras, click the 3 dots and then select your launcher thumbnail.

Now on the texture tab the place to upload your CAS thumb can either be under "configuration" or "Part Highlighting" so you just have to check around. Click the three dots and upload your CAS thumbnail.

If it's not makeup, then you can just export to sims3pack and be done with it. But I've heard that makeup is different and requires an extra step.

Edit > Project contents 

Find the PNG_PREVIEW and right click, go to renumber

Select group ID and click ok. Then export to sims3pack.

I have never personally made makeup, but all of the info I found points to this being the correct way so best thing to do is just try it out. No harm done.