What had I done wrong?

[Lancet]

I had learned how to do default replacement, so I tried to change the icons of accessories this time, and I found a very helpful tutorial in this thread (http://www.modthesims2.com/showthread.php?p=2066146&posted=1#post2066146).
When I opened the modified package with Sims2Pack Clean Installer, everything seems OK, and the thumbnail did show up.
However, in the CAS screen, the icon of that accessory I just altered remains the same, no sign of the thumbnail.
To be frank, I modified dozens of accessories, but none of them shows up the way I wanted, I must say that's very frustrating.

Can anyone help me on this?
I really love those gorgeous nail arts, and I don't want to give them up, but it really take a lot of time and efforts to go through them without proper thumbnails.

[Phaenoh]

You need to delete a file that stored the original thumbnails file. You prolly changed them correctly, but the game had an old copy it was still using. I think you need to delete the Thumbnails folder: C:\Documents and Settings\username\My Documents\EA Games\The Sims 2\Thumbnails

[Lancet]

Then which file should I delete?
I removed the one named " CASThumbnails.package", but I still cannot see the icons of those modified accessories.

By the way, I also deleted the "Group.cache" file before starting the game, but still, no icon shows up.

[Phaenoh]

I periodically delete the entire folder, the game takes a bit longer to load everything then the next time after, but that usually takes care of things.

[Lancet]

Oh~now I know what went wrong in the first place.
It turned out in 3IDR I must modify instance 1 ~ 10, not just the first two instances, only that the thumbnail will show up properly.
But thanks for your replies anyway, for you are the only person who responded to my question.