Have a look here:
Batch: Do something to every file in a folder
http://www.pptfaq.com/FAQ00536\_Batch-\_Do\_something\_to\_every\_file\_in\_a\_folder.htm
Substitute this for the Sub MyMacro section on that page:
Sub MyMacro(strMyFile As String)
' this gets called once for each file that meets the spec
' you enter in ForEachPresentation
' strMyFile is set to the path & file name each time
Dim oPresentation As Presentation
Set oPresentation = Presentations.Open(strMyFile)
' Change BMP to PNG or JPG or whatever
' Change 800, 600 to whatever width and height you want
' but make sure that it's proportional to your PPT slide size
With oPresentation.Slides(1)
.Export strMyFile & ".BMP", "BMP", 800, 600
End With
oPresentation.Close
End Sub