ToolboxBitmapAttribute memory leak?

Question

I'm seeing a possible memory leak with some code that I'm using to load an icon for a WF .NET Activity toolbox activity. A suggested method for retrieving the icon is to use reflection, however it appears that the reflection is causing the application to hang onto objects in memory, after closing the dialog. The symptom is basically the application size continues to increase (even after forcing a garbage collection) when opening and closing the dialog repeatedly.

Any ideas, or has anyone seen this before? The code is as follows:

    private void CreateToolboxBitmapAttributeForActivity(AttributeTableBuilder builder, Type activityType, Type activityImageType, string imageName)
    {
        if (activityImageType != null && !string.IsNullOrEmpty(imageName))
        {
            Bitmap bitmap = LoadIconImage(activityImageType.Assembly, imageName);

            if (bitmap != null)
            {     
                Type tbaType = typeof(System.Drawing.ToolboxBitmapAttribute);
                Type imageType = typeof(System.Drawing.Image);

                ConstructorInfo constructor = tbaType.GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new Type[] { imageType, imageType }, null);
                System.Drawing.ToolboxBitmapAttribute tba = constructor.Invoke(new object[] { bitmap, bitmap }) as System.Drawing.ToolboxBitmapAttribute;
                builder.AddCustomAttributes(activityType, tba);
            }
        }
    }

The Bitmap is loaded like this:

    private Bitmap LoadIconImage(Assembly a, string imageName)
    {
        Bitmap bmp = null;

        // attach to stream to the resource in the manifest
        Stream imgStream = a.GetManifestResourceStream(imageName);

        if (!(null == imgStream))
        {
            // create a new bitmap from this stream and 
            // add it to the arraylist
            bmp = Bitmap.FromStream(imgStream) as Bitmap;

            imgStream.Close();
            imgStream.Dispose();
            imgStream = null;
        }

        return bmp;
    }

As a point of reference, I'm basically using the approach detailed here: Displaying .NET Framework 4 Built-In Workflow Activity Icons in a rehosted Workflow Designer