5100MP Resolution changes when launching a slideshow in PowerPoint????

My computer outputs at UXGA (1600 X 1200) and the projector outputs this signal no problem (I know the native is 1400 X 1050 but it "upconverts" it I guess).  Anyway, even with the resolution set at SXGA+ or UXGA it drops to 1024 X 768 (XGA) when I start a slideshow in PowerPoint.  I have photos that are 1600 X 1200 in my presentations and want to show them at this resolution or at least at SXGA+.  Has anyone had this problem and if so how can it be fixed?? Thanks in advance....!