Use the HasButtonsCustom property to assign custom icons to the +/- signs on the parent items. The HasButtonsCustom property has effect only if the HasButtons property is exCustom. Use the Images, ReplaceIcon methods to add new icons to the control.

The following sample assigns different icons for +/- buttons:

With ExplorerTree1
    .BeginUpdate
    
    .BackColor = vbWhite
    .ForeColor = vbBlack
    .BackColorGroup2 = SystemColorConstants.vb3DShadow
    
    .Images "gBJJgBAICAAGAAEAAQhYAf8Pf4hh0QihCJo2AEZjQAjEZFEaIEaEEaAIAkcbk0olUrlktl0vmExmUzmk1m03nE5nU7nk9n0/oFBoVDolFo1HpFJpVLplNp1PqFRqVTqlVq1XrFZrVbrldr1fsFhsVjslls1ntFptVrtltt1vuFxuVzul1u13vF5vV7vl9v1/wGBwWDwmFw2HxGJxWLxmNx2PyGRyWTymVy2XzGZzWbzmdz2f0Gh0WjwEWH0r08ekEikgAkso184iGkicPjcU2+zjW4jTnZ++jbv4Tvjbu4fF48adAv5fI4XO4ka4fRjO+4G73UZ3na7IAgI="
    .Images "gBJJgBggAAwAAgACEKAD/hz/EMNh8TIRNGwAjEZAEXjAojJAjIgjIBAEijUlk8plUrlktl0vmExmUzmk1m03nE5nU7nk9n0/oFBoVDolFo1HpFJpVLplNp1PqFRqVTqlVq1XrFZrVbrldr1fsFhsVjslls1ntFptVrtltt1vuFxuVzul1u13vF5vV7vl9v1/wGBwWDjUPACAxGExVRwzAx0Sf4AxzAyWPxCAxeZouTyGdzmfx+cy+a0k5xuP0+Uw8ZzGVymc0uxmmcw2pjGt2G4x+y3ks2kO12ey2J3MY2G95GF4G/yO24u343D1vJ3m14HW5vX5eh7mv3fU2XMw+J0flxPY4PH8Gl0Xn7XZ+HjzGj23r5H09+d/Hx/f29b0OU+LQNU/0CwNA8EQSm6Kh8lUGo6j6QpGACSJPCqbsNBSVuAk5+Mik54B+lJwAfEYDxNFCTmBE8VAGlJgRdFsXxijRwRojJ4RujB8xKk8OI0gIA=="
    
    With .Groups.Add("Group")
        .BeginUpdate
            .Expanded = True
            .FullRowSelect = False
            .HasButtons = exCustom
            .HasButtonsCustom(False) = 1
            .HasButtonsCustom(True) = 2
            .LinesAtRoot = exLinesAtRoot
            Dim h As HITEM
            With .Items
                h = .AddItem("Item 1")
                .CellImage(h, 0) = 3
                .CellCaptionFormat(h, 0) = exHTML
                .InsertItem .InsertItem(h, , "SubItem 1"), , "SubItem 1.1"
                .InsertItem h, , "SubItem 2"
            End With
        .EndUpdate
    End With
    
    .EndUpdate
End With

Running the sample you get: