I've edited the umbrella and parasol that come with seasons so the first presets are more neutral (black/grey) and sims don't spawn the rainbow ones, but I can't figure out how to make the thumbnails that appear in build/buy mode and when editing with CASt reflect that change. TBH I'm not that bothered by the issue but I want to upload them here and I don't know if it will be acceptable.

For the thumbnails you see in the catalog (and when the object is in an inventory) you have to override
the THUM resources, I think the GroupId of the THUM is used to specify the preset (I'm not sure).

For instance, the umbrella has 12 THUM resources, 3 for every preset I guess (what changes is the size
of the image and the type of the resource).
The groupIds are 0x7800000X, where X is 0, 3, 4 and 5 (not sure why 1 and 2 are skipped).

Oh, the IID (Instance ID) of the umbrella's thumbnails is the same of the its OBJD.

Not sure about the previews you see using CASt, are those thumbnails too (the bigger ones maybe?)
or are automatically generated?

EDIT: just noticed you mentioned TextureTweaker, I don't know about that. I answered with S3PE in mind.

Thank you
I used S3OC to clone the umbrella & parasol, unchecked the 'renumber internally' box to make it an override, but there are no THUM (that should be under the tag column, right?) resources in the package file when opening it with s3pe, even before making any changes with texture tweaker.
The thumbnails I'm referring to are these in the top left corner, the first image should show a black umbrella. Isn't there a way to make the game auto generate a thumbnail that can be saved into the .package? I really have no clue what I'm doing. eek.

Thumbnails generated by the game are saved in
<SOMEWHERE>/Electronic Arts/The Sims 3/Thumbnails/CASThumbnails.package (for CAS stuff)
<SOMEWHERE>/Electronic Arts/The Sims 3/Thumbnails/ObjectThumbnails.package (for objects)
But the game generates them only if they don't exist. Those for umbrella already exists.

When cloning with S3OC did you tick the checkbox "Include thumbails"?

You can find them "manually" with S3PE. Check the IID of the umbrella OBJD and search THUM
resources with that same IID (they should be in the Seasons EP installation directory inside the
Thumbnails folder).

Bye ^^

Oops I forgot to check 'include thumbnails' the first time, however I tried a second time (this time including the thumbnails) and yep the THUM files showed up in the package, but I've decided to keep the catalog thumbnails as they are seeing as I'm not changing the mesh or anything and people are probably used to seeing that image of the umbrella/parasol to grab it out of the catalog. But I'm still having the issue of the preset images not being updated and that's what I can't figure out how to fix.

Those images probably use the thumbnails too. Try changing one of the thumbnail, just as a test.