Thank you for your rapidity, I'm on Host Mutualist 1and1 and I don't know how to view server's logs.
In the admin panel, I have these informations:
System info
You have PHP version: 5.2.17
GD Library version:
Array
(
[GD Version] => bundled (2.0.34 compatible)
[FreeType Support] => 1
[FreeType Linkage] => with freetype
[T1Lib Support] =>
[GIF Read Support] => 1
[GIF Create Support] => 1
[JPG Support] => 1
[PNG Support] => 1
[WBMP Support] => 1
[XPM Support] =>
[XBM Support] => 1
[JIS-mapped Japanese Font Support] =>
)