SetWatermarkText{includegraphics{filename}}
SetWatermarkAngle{0}
SetWatermarkScale{0.45}
SetWatermarkColor[rgb]{0,1,0}
SetWatermarkLightness{1}

documentclass{article}

usepackage[firstpage]{draftwatermark}
usepackage{tikz}
SetWatermarkText{tikz{node[opacity=0.2]{includegraphics{example-image-a}};}}
SetWatermarkAngle{0}
SetWatermarkScale{0.45}
SetWatermarkColor[rgb]{0,1,0}
SetWatermarkLightness{1}

usepackage{lipsum}

begin{document}

lipsum[1-10]

end{document}

documentclass{article}
usepackage[a6paper]{geometry}% just for the example
usepackage[firstpage=true]{background}
usepackage{lipsum}% just to generate text for the example

backgroundsetup{
contents={includegraphics{example-image-a}},
angle=0,
scale=0.5,
color=black,
opacity=0.2
}

begin{document}

lipsum[1-3]

end{document}

documentclass[12pt]{article}
usepackage[{firstpage]{draftwatermark}
usepackage{tikz}
SetWatermarkText{tikz{node[opacity=0.4]{includegraphics{LTH_C_RGB.png}};}}
SetWatermarkAngle{0}
SetWatermarkScale{1}
SetWatermarkColor[rgb]{0,1,0}
SetWatermarkLightness{1}
usepackage[english]{babel}
usepackage{natbib}
usepackage{url}
usepackage[utf8x]{inputenc}
usepackage{amsmath}
usepackage{graphicx}
graphicspath{{images/}}
usepackage{parskip}
usepackage{fancyhdr}
usepackage{vmargin}
setmarginsrb{2.5 cm}{1 cm}{2.5 cm}{2.5 cm}{1 cm}{1.5 cm}{1 cm}{1.5 cm}