By default, Windows XP opens all picture files (gif,jpg,...) with the included Windows Picture and Fax Viewer no matter what other picture viewers you have installed. To disable the Windows Picture and Fax Viewer, unregister shimgvw.dll. This can be done from command prompt:
regsvr32 /u shimgvw.dll

(Additional note: unregistering shimgvw.dll will also disable thumbnail view in Explorer and may cause problems changing user account and desktop background pictures. To reenable Picture and Fax viewer again type:  regsvr32 shimgvw.dll) yoyo