Difficult? That depends on who you ask :)

If you're more comfortable with customizing CSS I would look at Dang's approach. If you're a developer and want to play with some code I would look at working with an ASCX file.

Nice problem BTW, I couldn't resist

In your ASCX add this on top:

<script runat="server">
    Private Function GetSeason() As String
        'Returns the current Season
       
        Dim dDate As Date = Now() ' change Now() to "2008-3-21" , to test other dates
       
        Dim decDate As Decimal = dDate.Month + dDate.Day / 100
        Dim sSeason As String = String.Empty
       
        If decDate > 3.2 And Not decDate > 12.2 Then
            If decDate > 6.2 Then
                If decDate > 9.22 Then
                    sSeason = "Fall"
                Else
                    sSeason = "Summer"
                End If
            Else
                sSeason = "Spring"
            End If
        Else
            sSeason = "Winter"
        End If
       
        Return sSeason
    End Function
   
</script>

This would be your Seasons Logo:

<img src="<%=SkinPath%>Logo<%=GetSeason()%>.png" alt="Season Logo" />