It's quite straight forward to change the texture:
Lua Code:
local CPoints = {}
for index = 1, MAX_COMBO_POINTS do
local CPoint = self:CreateTexture(nil, 'BACKGROUND')
CPoint:SetTexture[[Interface\AddOns\Path\To\Texture]]
-- Position and size of the combo point.
CPoint:SetSize(16, 16)
CPoint:SetPoint('TOPLEFT', self, 'BOTTOMLEFT', index * CPoint:GetWidth(), 0)
CPoints[index] = CPoint
end
-- Register with oUF
self.CPoints = CPoints