I am seeing the same thing. This seems to be a bug.
I changed the LANDesk ReportHeaderImage to my Company image in the Portrait and Landscape Report templates in LDMS96sp2.
I tried using .jpg and .png images in the templates. The reports that run and display in portrait format display my image in the header. The reports that display in landscape format display the LANDesk image in the header. The Embedded image header properties are identical for both the Portrait and Landscape templates, yet the landscape reports still display the LANDesk image in the header.
In summary the reports that display in "Portrait" work and display my company header, the Reports that display in "Landscape" don't work and always display the default LANDesk header.