UserForm1.Hide

If ( 10  <= X And X <= Image1.Width -  10 ) And ( 10  <= Y And Y <= Image1.Height -  10 ) Then
    UserForm1.Show
Else
    UserForm1.Hide
End If

UserForm1.ShowModal = False

Private Sub Image1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
If ( 10  <= X And X <= Image1.Width -  10 ) And ( 10  <= Y And Y <= Image1.Height -  10 ) Then
    ' запуск некой функции
Else
    UserForm1.Hide
End If
End Sub
...
Private Sub Image2_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)...
...
Private Sub Image400_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)

Public WithEvents ImageGroup As msforms.Image
Private Sub ImageGroup_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
If ( 10  <= X And X <= ImageGroup.Width -  10 ) And ( 10  <= Y And Y <= ImageGroup.Height -  10 ) Then
    UserForm1.Image1.Picture = ImageGroup.Picture ' это хз может не работать, но... идея такая
    UserForm1.Show
Else
    UserForm1.Hide
End If
End Sub

Option Explicit
Public MyImages( 1  to  400 ) As New ImageMyMoveClass
Public Sub MyStart()
Set MyImages( 1 ) = Image1
Set MyImages( 2 ) = Image2
'...
Set MyImages( 400 ) = Image400
End Sub

Private Sub Workbook_Open()
MyStart
End Sub

Лист1.Shape("Image1").Select 'как-то так

Option Explicit
Public MyImages( 1  to  400 ) As New ImageMyMoveClass
Public Sub MyStart()
Dim i as Integer
For i =  1  to  400 
     Set MyImages(i) = Application.ActiveSheet.OLEObjects.Item("Image" & CStr(i))
Next
End Sub