Hm, maybe the NormalTexture is not meant to be used like this for action bar buttons.
I found this work around:
Code:
local myButton = CreateFrame("CheckButton", nil, UIparent)
myButton:SetPoint("CENTER")
myButton:SetSize(50, 50)
myButton.normalTexture = myButton:CreateTexture()
myButton.normalTexture:SetAllPoints()
myButton.normalTexture:SetDrawLayer("BACKGROUND", 0)
myButton.normalTexture:SetTexture(132224)
myButton:SetHighlightTexture("Interface\\Buttons\\ButtonHilight-Square")
myButton:SetPushedTexture("Interface\\Buttons\\UI-Quickslot-Depress")
myButton:SetCheckedTexture("Interface\\Buttons\\CheckButtonHilight")