% define a lua macro to print graphic
% #1 = filename stem
% #2= figure width
% #3 = figure height
% #4 = graphic type: pdf, png, etc.
\def\luagraphic#1#2#3#4{%
\directlua{
a = img.new({filename="#1.#4",width="#2",height="#3"})
img.write(a)
}}

hi
\luagraphic{example-image-a}{2cm}{3cm}{pdf}
\bye