Change the appearance of a single hyperlink in PowerPoint 2010

This little macro will do the job for you:

Sub AddPictureBorders()
' Before running this, apply the style you want to one picture
' then use the formatting paintbrush to pick up the formatting
' This will apply the formatting to each picture.
Dim oSh As Shape
Dim oSl As Slide

For Each oSl In ActivePresentation.Slides
    For Each oSh In oSl.Shapes
        With oSh
            ' ignoring linked pictures, OLE objects
            ' and such
            If .Type = msoPicture Then
                .Apply
            End If
        End With
    Next
Next

End Sub

On Windows, Powerpoint uses a different color for followed hyperlinks (changes color after the link is selected). My guess is that the color theme that you're using has the same color the hyperlink and followed hyperlink. No experience with Macs but here's some info about how this works on Windows.

With powerpoint on Windows here's how you edit the colors used for hyperlinks.

Select the Design tab and click on Colors. From here you can select a template that uses different colors for the hyperlink and followed hyperlink. If none exist, create your own template. Note that you cannot edit the built-in themes. To edit custom themes, right-click on the theme and select Edit...