OK, make sure you have set the preferred width for the nested table and its column to however many inches/centimeters you require.
Then, for the cell, check the 'fit text' property. This will stop the cell expanding but may result in parts of pictures that are wider than the cell when fitted to the row height disappearing. The following macro will correct such images after they're inserted.
You can select either a single cell or a whole table for processing - which means to can leave the resizing till the end.
Fiddling with the 'fit text' property isn't really necessary, but it prevents the rest of the parent cell's layout being messed with before the macro is run.
Code:
Sub FitPics()
Dim Tbl As Table, TblCl As Cell, Cell As Cell, i As Long, sWdth As Single
With Selection
If .Information(wdWithInTable) = False Then Exit Sub
For Each Cell In .Cells
With Cell
sWdth = .PreferredWidth
For i = 1 To .Range.InlineShapes.Count
With .Range.InlineShapes(i)
.LockAspectRatio = True
If .Width > sWdth Then .Width = sWdth
End With
Next
End With
For Each Tbl In Cell.Tables
For Each TblCl In Tbl.Range.Cells
With TblCl
sWdth = .PreferredWidth
For i = 1 To .Range.InlineShapes.Count
With .Range.InlineShapes(i)
.LockAspectRatio = True
If .Width > sWdth Then .Width = sWdth
End With
Next
End With
Next
Next
Next
End With
End Sub