I am presenting a poster at a conference and need to include a figure as background information. Given that it is properly cited, is it okay to include a figure I did not make in an academic poster for a conference? Should posters contain only figures made by the presenter? Would it be seen as unprofessional?
If there's something essential in the figure that you're including that you and your research group can't reproduce on your own, then it's justifiable to include a plot or other figure produced by someone else and already published in the literature. However, such usage should be restricted to only what is essential for your poster—you should never use others' work merely to "fill out" your poster.
It might be legally defensible (via a Fair Use defense at trial in the US), but without a license or other permission from the copyright holder, it's a copyright violation in the US. Whether it comes from a journal or conference publication where the publisher owns the copyright or some other venue where the authors retained the copyright, you should ask for permission. Most journals have an online mechanism for requesting permission to reuse something they published (e.g. IEEE's Permission to Reuse IEEE material page links to their copyright permission request page where they have partnered with a clearance firm).
You should do this rather than ignoring the problem. Do not simply assume that citing the original publication is enough or that Fair Use (in the US) is enough to protect you. You could get sued and face an expensive lawsuit that you might win anyway. You would still have to pay a lawyer to defend you. I don't know of any cases where a publisher has sued and lost or won over reproduction of copyright-protected material that was either ruled a fair use or not, in the US, but I assume those cases are out there.
No matter what you do, you also must cite the original source. But, please, please, please, get permission. My understanding is that permission is routinely given, though it may take a little while to receive the required sign off.