1) Get a pair of "drafting dividers" ... they are a high quality
"compass" but both ends are metal points. Very good for
transfering measurements - and you will find new ways to use
them for a long time.
2) If you don't have one of the very affordable digital calipers
get one. You want one that allows you to switch between
metric and english at the push of a button (I haven't seen
one that doesn't but it is a key feature).
3) If the picture(s) you are working from aren't fairly square
you are going to have to carefully work everything out.
However - it is rare these days to need to use photos for
dimensional data ... for subjects in our interest range. If you
have a pic and you want 'plans' for it ... usually it is as close
as a Google search. Another great resource is "old hobby mags".
A third way is to ask questions on this list ... often there is
someone who has already done that car! Many kits include plans -
if the car in question has ever been produced ...
If you are working on a car that is earlier than about 1930
you are more likely to have to develop your own measurements ...
P.S. I know, I know ... you already have a usable pic so why go
do the research? You may find that when doing the research
you will learn other stuff you didn't know and end up with a
better model! Besides when doing the research you are likely
to find your self "humming a happy tune" ...