Is it unprofessional to use graphics from Wikipedia in a poster?
The primary issues with using Wikipedia for academic research are that it's a tertiary source, and there's no credibility/quality assurance.

So, you should make sure that


If the image contains intellectual content that requires citation, you should cite a primary source for that content. 
The image (including its factual/intellectual content) meets academic standards of quality and accuracy.


Assuming these are satisfied, reusing images from Wikimedia (or a similar source) is not inherently unprofessional. Of course, if the image is of poor quality or doesn't fit in well with your poster, it will look unprofessional - but this would also be true if you had created the image yourself.

This is, of course, assuming that the image you are using is in Wikimedia commons (not all images on Wikipedia are), and


you follow the license requirements (protects against legal/copyright problems)
you correctly attribute the source (protects against ethical/plagiarism concerns)