In case someone still finding the answer,
You can,
in .h:
UTexture2D buttonTexture; // maybe set it somewhere in your UWG editor..
…
in .cpp
UButton button = (UButton *) yourUMG->GetWidgetFromName(TEXT(“ButtonName”));
if (button)
{
FSlateBrush slatebrush;
slatebrush.SetResourceObject(buttonTexture);
FButtonStyle bnStyle = button->WidgetStyle;
bnStyle.SetNormal(slatebrush);
button->SetStyle(bnStyle);
}