it's probably in your cache.
imvu will strip the '.tga' extension, but it's still a .tga file.
you can use an app like irfanview to search the cache for files that include the product ID #.
then, check the create date of that file, and search again for all files in the cache created on that same date.
this will (hopefully) narrow it down to a small number of files. you can preview them with irfanview and find the one you're looking for.
hth !