[TYPO3] No thubnails in BE over a certain image-size? Why?

[Johannes Konert]

Hi,
maybe it's an easy thing, but haven't found a former question similar to 
this:

In BE I can select images to attach to records like in "text with 
images" (that you know what thubnails I mean).
There comes a popup showing the filelist within fileadmin-folder.
For most folders I get tumbnails of the pictures, nice.
But for some images I do not get thubnail-preview.

The images that work are about 50 KB big,
the others are over 200KB.

Is there somewhere a config-option (TS?) where something like
image.maxSizeForThubnailPreview = 50KB (or similar) is set?

I want tumbnails for all images, even if they are 2MB, if possible....
but I cannot find the reason why there are only thubnails of some jpgs...

Thanks for your hints on this maybe trivial question.
Regards
Johannes