La clé est bien celle là


HKEY_CLASSES_ROOT\jpegfile\shell\open\command


voila l'extrait
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\jpegfile]
@="Image JPEG"

[HKEY_CLASSES_ROOT\jpegfile\CLSID]
@="{25336920-03F9-11cf-8FD0-00AA00686F13}"

[HKEY_CLASSES_ROOT\jpegfile\DefaultIcon]
@="C:\\Program Files\\Internet Explorer\\iexplore.exe,8"

[HKEY_CLASSES_ROOT\jpegfile\shell]

[HKEY_CLASSES_ROOT\jpegfile\shell\open]

[HKEY_CLASSES_ROOT\jpegfile\shell\open\command]
@="\"C:\\Program Files\\Internet Explorer\\iexplore.exe\" -nohome"

[HKEY_CLASSES_ROOT\jpegfile\shell\open\ddeexec]
@="\"file:%1\",,-1,,,,,"

[HKEY_CLASSES_ROOT\jpegfile\shell\open\ddeexec\Application]
@="IExplore"

[HKEY_CLASSES_ROOT\jpegfile\shell\open\ddeexec\Topic]
@="WWW_OpenURL"

regarde encore si tu n'as rien qui ressemble