How to reset all rulers to default across multiple pages in a document?

The "ruler settings" are really the paragraph indents, which could be all the same (as they are initially) or different for some paragraphs or even for each individual paragraph.

Even if you have only pictures and no text in the document, you still have paragraphs -- represented by "paragraph marks" that appear as ¶ symbols when you display nonprinting characters (click the ¶ button on the Home tab to toggle them on and off).

If you select more than one paragraph at a time, changes you make to the indents by dragging the markers on the ruler will apply to all the selected paragraphs. You can select all the paragraphs in the document body (but not headers, footers, footnotes, text boxes) by pressing Ctrl+A (the Select All command).

If all the paragraphs have the same paragraph style (usually Normal style) and if the indents are the only paragraph formatting you've applied, then the quickest way to reset all the indents to the style's definition is to press Ctrl+A and then Ctrl+Q (the Reset Paragraph command). If there are other changes that you want to keep, though, press Ctrl+A and use the ruler.

Unfortunately, there is no similarly simple way to set all the pictures to the same height. It could be done with a macro like this one (see http://www.gmayor.com/installing_macro.htm):

Sub PictureHeights()
    Const NewHeight = 2  ' value in inches
    Dim sh As Shape
    Dim ish As InlineShape
   
    For Each sh In ActiveDocument.Shapes
        With sh
            .LockAspectRatio = msoTrue
            .Height = InchesToPoints(NewHeight)
        End With
    Next
   
    For Each ish In ActiveDocument.InlineShapes
        With ish
            .LockAspectRatio = msoTrue
            .Height = InchesToPoints(NewHeight)
        End With
    Next
End Sub