s00mb said:
if there was an upper bound on how much energy you can pack into a photon
It depends on what kind of upper bound you mean.
Any photon is going to have to come from some process that emits energy, and how much energy can be packed into that photon is going to depend on how much energy the process that emits it can produce and pack into one photon. I don't know how much energy the most energetic photons we've ever observed from an actual process had, but it will be some finite number.
Also, if a photon has enough energy and is in the presence of other particles, it can create an electron-positron pair--or, if it has even more energy, other particle-antiparticle pairs. (Note that in vacuum this can't happen--a single photon can't create a pair, because there is no way to conserve both energy and momentum. It takes two photons to create a pair in vacuum. But in the presence of other particles that can recoil in response to the pair creation, a single photon can do it.) So once you get to high enough energies, you can't really think in terms of just photons any more, because other particles can be created.
Theoretically speaking, we know that at high enough energies, electroweak symmetry is no longer broken, and if electroweak symmetry is not broken, there are no "photons" in the sense we use the term; the set of gauge bosons for the electroweak interaction is different. So it wouldn't make any sense to think of a "photon" having energies that high because at that high an energy the theory that describes things in terms of "photons" wouldn't apply.
s00mb said:
if there was not one wouldn't that mean that you could create an extremely powerful electric and magnetic field with just a single photon?
No, because the kinds of things we call electric and magnetic fields are made up of huge numbers of photons. The phenomena associated with single photons are very different from the phenomena associated with electric and magnetic fields.