Broken original images after upload

Hello,
I have the Problem that when I´m upload pictures to my gallery installation some images are broken.
If I upload the same pictures repeatedly to a gallery always different pictures are broken.
I also used different upload funktions like webdav, upload applet, gallery remote and the windows gallery printer.
When I am viewing the pictures after the upload on the server the original is broken (full size). Also the thumbnails of course.
I also used different Graphics Toolkits like GD, NetPBM, ImageMagick and follow the instuctions of the FAQ Thumbnails / resized images don't get generated properly, how can it be fixed?
Nothig is working.

Here is a typical broken picture:

[img]http://gallery.menalto.com/files/IMG_0782.jpg[/img]

Is there a resizing of the original pictures after upload?
Or have anyone an idea?

Thanks

Juergen

System Information:

Gallery URL = http://192.168.178.25/main.php
Gallery version = 2.3.1 core 1.3.0.1
API = Core 7.54, Module 3.9, Theme 2.6, Embed 1.5
PHP version = 5.2.6-1+lenny4 apache2handler
Webserver = Apache/2.2.9 (Debian) PHP/5.2.6-1+lenny4 with Suhosin-Patch mod_ssl/2.2.9 OpenSSL/0.9.8g
Database = mysqli 5.0.51a-24+lenny2+spu1, lock.system=flock
Toolkits = Getid3, LinkItemToolkit, Thumbnail, ArchiveUpload, Dcraw, ImageMagick, jpegtran, Ffmpeg, Exif
Acceleration = none, none
Operating system = Linux VdrServer 2.6.28-1+etobi.1-486 #1 Mon Feb 23 23:02:06 UTC 2009 i686 (debian system)
Default theme = matrix
gettext = enabled
Locale = en_GB
Browser = Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.2) Gecko/20100115 Firefox/3.6 (.NET CLR 3.5.30729)
Rows in GalleryAccessMap table = 32
Rows in GalleryAccessSubscriberMap table = 199
Rows in GalleryUser table = 3
Rows in GalleryItem table = 197
Rows in GalleryAlbumItem table = 4
Rows in GalleryCacheMap table = 0