How do radio signals contribute to gravity? First, the inspiration for this question:
I just read that it takes one hour to send a picture from the New Horizons space probe, to Earth. It also takes around 5 hours for that picture to reach earth.
This means that there are about 4 pictures "floating" in space, somewhere between Pluto and earth. 
This information is energy, and energy contributes to gravitation somewhere between Pluto and Earth at this moment.
Is this last assertion correct?
If so:
The sun outputs $10^{26} \text{ watt}$ of energy. It's been doing that for more than 4 billion years. In the observable universe, there are between $10^{22}$ and $10^{24}$ stars.
If the average star is similar to the sun, and we ignore energy that has been stored in planets, then up to $10^{(26 + 24)} = 10^{50} \text{ watt}$ of energy is being added to "empty space".
4 billion years is something like $10^{17}$ seconds (rounded down).
This means that there might be as much as $10^{(50+17)} = 10^{67} \text{ J}$ of energy, just moving about the universe.
Is this negligible?
The amount of energy is probably much higher than $10^{67}\rm\,J$, as I do not take into account light that was emitted more than 4 billion years ago. It also does not take into account stars beyond our observable universe.
I apologize for using very simplified estimates, but since the numbers are so big the errors become less important. I have naively used $E=mc^2$ to go from joules to kilograms. I get something like $10^9$ Milky Way masses.
Is this energy taken into account, when scientists say that the universe is expanding? I have a feeling that this energy would make it appear as if the universe is expanding faster.
 A: There absolutely is a contribution to the energy density of the universe due to radiation.  It's small compared to baryonic matter, dark matter, or dark energy, and is mostly due to the cosmic microwave background (CMB) left over from the Big Bang.  Sure, the CMB is faint here compared to the Sun, and faint within the Galaxy compared to the light from nearby stars; but the CMB fills the entire volume of intergalactic space more or less uniformly, while starlight from  galaxies peters out like $1/r^2$ as you move into the void.  Volume-averaged, the CMB wins.
The Particle Data Group quotes number densities
\begin{align}
n_\text{photon} &= \rm 410.7 \,cm^{-3} \\
n_\text{baryon} &= \rm 2.482\times10^{-7}\,cm^{-3}
\end{align}
for CMB photons and baryons — that is, there's about two baryons for every three billion photons.  But baryons (protons and neutrons) each have a rest energy
\begin{align}
m_\text{baryon}c^2 &=  940\,\text{MeV}
\end{align}
while the CMB photons have energy
\begin{align}
E_\text{photon} &= k T_\text{CMB} = 86\, \frac{\mu\rm eV}{\rm K} \cdot 2.7\,\rm K
\\ & = 230 \rm\,\mu eV.
\end{align}
This energy ratio $E_\text{baryon} / E_\text{photon} \approx 4\times 10^{12}$ counterbalances the number density ratio $n_\text{photon}/n_\text{baryon} = 6\times10^{-10}$, so that the energy density of the universe due to baryons is about 70 times larger than the energy density due to radiation.  Since the baryons only make up about 5% of the energy density anyway, it's safe to leave the radiation density off in most discussions.
You've computed $10^9$ Milky Way masses' worth of stellar radiation emitted in the last third of the age of the Universe.  If we guess there are about $10^{14}$ Milky Ways out there, my estimates give about $10^{12}$ Milky Ways worth of cosmic microwave background photons, which leaves plenty of room for your starlight to get lost in the noise.
So, your assertion is correct!  Your arithmetic looks plausible!  But the effect can be neglected.
A: 
Is this energy taken into account, when scientists say that the universe is expanding? 

Yes and no. Firstly, we see the universe is expanding rather directly, and try to figure out how many stars there are and how much they are putting out and use the different to figure out dark energy and dark matter. So when we estimate dark matter and dark energy, we take that energy into account.
But we'd pretty much how much the universe is expanding by just looking at it and measuring it rather directly.
