  In a set of two papers, we propose to study an old-standing problem, namely
the electromagnetic interaction for particles of arbitrary spin. Based on the
assumption that light-cone helicity at tree level and $Q^2=0$ should be
conserved non-trivially by the electromagnetic interaction, we are able to
derive \emph{all} the natural electromagnetic moments for a pointlike particle
of \emph{any} spin. In this first paper, we propose a transparent decomposition
of the electromagnetic current in terms of covariant vertex functions. We also
define in a general way the electromagnetic multipole form factors, and show
their relation with the electromagnetic moments. Finally, by considering the
Breit frame, we relate the covariant vertex functions to multipole form
factors.
