[Typo3] Thumbneils problem.

[Meli Marco]

Hi,
I'm new and I have just installed Typo3 src-3.8.0 & quickstart-3.8.0 on my
FC4.
It seems is ok, except I can't see the thumbneils of any images I uploaded.
I can see images thumbneils's comes with typo installation in
"Image_Archive", but if I download ones of them and upload it again in
"user_upload", no tmb* files are created in typo3temp dir.
Also if I delete cache or tmb* files manually, typo recreate tmb images of
images default installed only.
Anyway my images are correctly displayed in thumbneils on "Page" section and
web page.The problem seems to be in "Filelist" and "Images" section.
If it could be help icons with "No thumb generated" is displayed. 
Thanks.