Dialog_pickfile problem

Hi,

I am using Dialog_pickfile to write a jpeg, using file='a string i
insert here'. My problem is that if i want to change the directory in
which dialog_pickfile defaults to  it wipes the string i initially
inserted. Is there a way i can fix the default filename so that the
user can pick a directory to save the jpeg without having to retype
the file name back in?

filename = Dialog_Pickfile(/Write, File=uname
+'-'+info.file_name,DEFAULT_EXTENSION='jpeg')
IF filename NE '' THEN Write_JPEG, filename, snapshot, True=1

many thanks,

Clive