Just if someone need the solution:
<TypeConverter(GetType(ImageKeyConverter))> _
<RelatedImageList("LoadingImageList")> _
<Editor("System.Windows.Forms.Design.ImageIndexEditor, System.Design", GetType(Drawing.Design.UITypeEditor))> _
<Description("Imagen de carga que se mostrará en el control cuando se indique.")> _
Public Property LoadingImageKey() As String
Get
Return sPic
End Get
Set(ByVal value As String)
sPic = value
End Set
End Property
And for adding the GIFs images into the ImageList, I added them to the resources and load them into the Imagelist on the creation of the control. I have a secondary var to hold the GIFS and the images are related with a string key:
Private Sub SysInfoControl_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim result As Resources.ResourceSet = My.Resources.ResourceManager.GetResourceSet(System.Globalization.CultureInfo.CurrentCulture, True, True)
If pics.Count = 0 Then
For Each elem As DictionaryEntry In result
If elem.Value.GetType Is GetType(Bitmap) Then
pics.Add(elem.Key.ToString(), CType(elem.Value, Bitmap))
ImageList1.Images.Add(elem.Key.ToString(), CType(elem.Value, Bitmap))
End If
Next
If pics.ContainsKey(sPic) Then picBoxLoading.Image = pics(sPic)
End If
End Sub
Example of the result: