  In this paper we study identities and images of polynomials on null-filiform
Leibniz algebras. If $L_n$ is an $n$-dimensional null-filiform Leibniz algebra,
we exhibit a finite minimal basis for $\mbox{Id}(L_n)$, the polynomial
identities of $L_n$, and we explicitly compute the images of multihomogeneous
polynomials on $L_n$. We present necessary and sufficient conditions for the
image of a multihomogeneous polynomial $f$ to be a subspace of $L_n$. For the
particular case of multilinear polynomials, we prove that the image is always a
vector space, showing that the analogue of the L'vov-Kaplansky conjecture holds
for $L_n$. We also prove similar results for an analog of null-filiform Leibniz
algebras in the infinite-dimensional case.
