You will need to use controls collection but controls arrays are much easier to manage (disregard all math stuff - did it for a fun):
VB Code:
Private Sub Command1_Click()
Dim i As Integer
On Error Resume Next
For i = 1 To 3
Controls("Picture" & i).BackColor = Sqr(i * 777 ^ 3)
Next i
End Sub
'OR
Private Sub Command1_Click()
Dim ctl As Control, i As Integer
On Error Resume Next
For Each ctl In Me.Controls
If TypeOf ctl Is PictureBox Then
ctl.BackColor = Sqr(i * 777 ^ 3)
End If
i = i + 1
Next ctl
End Sub