my szs tools support tpl/tex0/breft images with different image formats including cmpr and mipmaps/multimage support.
Code:
# convert to png:
wimgt dec SOURCEIMAGE --dest output.png
# convert to tpl/cmpr
wimgt enc SOURCE.png --dest output.tpl --transform tpl.cmpr
One issue: There are different tpl/tex/... sub formats and my tools only support the Mario Kart related formats.