Here is a button that you can use that will import a file into a global parameter called myData:

Button["Import Data",
 myData = Catch@Module[{filePath},
    filePath = SystemDialogInput["FileOpen"];
    If[filePath === $Canceled, Throw[$Failed]];
    Import[filePath]
    ],
 Method -> "Queued"
 ]

The Method -> "Queued" option keeps the process from timing out since dynamic behaviors have a time limit on them when they are preemptive (as buttons are). The Catch and Throw just help to keep things nice if you cancel the system dialog.

.... seems not to work anymore ...

I would just open a file (picture) with a button, but I don't understand why my code doesn't work ...

Button["Click Here",
 Import[SystemDialogInput["FileOpen"]],
 Method -> "Queued"
 ]

Thanks for an help ...

Yvan

Many thanks Gustavo It works ! The best new of the day for me today. So that I can go to sleep earlier today !