The picture insertion behaviour is primarily determined by your Word options. Under File|Options|Advanced>Cut, Copy & Paste>Insert/Paste pictures as, you can choose 'In-line with text' or one of the floating formats. Only the 'In-line with text' option will give you the behaviour desired for your macro. Even with that option, however, if you drag & drop an image that's in a 'floating' format from another document, or from another location in the same document, the format won't change to 'In-line with text' at the destination.
Try the following revision to your code. Do note that any Shape objects that get converted to InlineShapes are liable to change position to wherever they are anchored.
Code:
Sub AfbeeldingImporteren()
Dim i As Long
With Selection
'Convert floating shapes to inline
For i = .ShapeRange.Count To 1 Step -1
.ShapeRange(i).ConvertToInlineShape
Next
'process all InlineShapes
For i = 1 To .InlineShapes.Count
With .InlineShapes(i)
' retain aspect ratio
.LockAspectRatio = msoTrue
' set width to 225 px
newWidth = 225
.Width = newWidth
' Add Borders
With .Line
.Weight = 0.75
.DashStyle = msoLineSolid
.Visible = msoTrue
.ForeColor.RGB = RGB(0, 0, 0)
End With
End With
Next
End With
End Sub