For this you should be using a userform of the type discussed in the article
"How to create a Userform" at:

http://word.mvps.org/FAQs/Userforms/CreateAUserForm.htm

Then you need a button on the form with a caption something like "Browse for
Image" and for the Click() event for that form, you would use the following
code:

   With Dialogs(wdDialogInsertPicture)
       If .Display Then
           'Populate the txtOrgChartPath control with the selected filename
           txtOrgChartPath = WordBasic.FilenameInfo$(.Name, 1)
           cmdInsertOrgChart.Enabled = True
       End If
   End With

In the form from which I grabbed this piece of code, the user was browsing
for a file that contained and organization chart and there was a textbox
(txtOrgChartPath) on the form that was populated with the path and filename
of the file that was selected by the user when the InsertPicture dialog was
displayed.  The code also activated a button that when clicked, inserted the
organisation chart into the document using the following code:

Private Sub cmdInsertOrgChart_Click()
'   Check that a file has been selected
   If InStr(txtOrgChartPath, ":") = 0 Then
       MsgBox "You must select a file for the Organization Chart."
       Exit Sub
   End If
'Insert organization chart into Quality Manual
   Set myDoc = Documents.Open(PathofSystemFiles & "\Quality
Manual\Quality_Manual.doc")
   If myDoc.Bookmarks.Exists("Org_Chart") = True Then
'Clear existing organization chart
       myDoc.Bookmarks("Org_Chart").Range.Tables(1).Cell(1, 2).Range.Delete
'Insert new organization chart
       myDoc.Bookmarks("Org_Chart").Range.Tables(1).Cell(1,
2).Range.InlineShapes.AddPicture Filename:=txtOrgChartPath
'Save and close the document
       myDoc.Save
   End If
   myDoc.Close

End Sub

You will need to modify this for your situation, but it should show you the
necessary syntax to use.

Doug's approach is one way. Another is to include a MACROBUTTON field in the
document, like this

{ MACROBUTTON BrowseForPicture Double-click to insert a picture}, where the
{ } characters are field braces inserted by pressing Ctrl-F9.

Then the BrowseForPicture macro can display the Insert Picture dialog using
code based on what Doug has provided. To insert the picture into the
document, you may need to have the code temporarily unprotect the document,
insert the picture, and then re-protect the document.