The [primary issues][1] 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][2]), and

 * you follow the [license requirements][3] (protects against legal/copyright problems)
 * you correctly attribute the source (protects against ethical/plagiarism concerns)


  [1]: http://academia.stackexchange.com/questions/19083/are-there-instances-where-citing-wikipedia-is-allowed
  [2]: http://en.wikipedia.org/wiki/Wikipedia:Images
  [3]: http://commons.wikimedia.org/wiki/Commons:Reusing_content_outside_Wikimedia