It's easy enough to use a macro to format all the images so they're 5cm less than the page width or the margin width, then centre them horizontally on the page. For example, if the screen shots have been inserted as floating images:
Code:
Sub Demo()
Application.ScreenUpdating = False
Dim i As Long, sWdth As Single
With ActiveDocument.Range
With .Sections(1).PageSetup
sWdth = .PageWidth - .LeftMargin - .RightMargin - .Gutter - CentimetersToPoints(5)
End With
For i = 1 To .ShapeRange.Count
With .ShapeRange(i)
.LockAspectRatio = True
.Width = sWdth
.RelativeHorizontalPosition = wdRelativeHorizontalPositionMargin
.Left = wdShapeCenter
End With
Next
End With
Application.ScreenUpdating = True
End Sub
The same kind of thing can be done with screen shots that have been inserted in-line with the text but, depending on what else is in the paragraphs they're attached to, it might mess with some of your formatting.