A: Your use of an image, no mater the source (online journal, web site, UpToDate or other resource), must constitute fair use, as described here. Content created by the federal government is not copyright protected, but if the government is reusing images with permission, that permission does not extend to other users.
If you are using the image in a presentation, appropriate credit indicating the original source should appear on the slide and any handouts.
Brown. N Engl J Med 2009; 360: 2615
UpToDate Online 17.1
Menkes. Child Neurology (2006)
If you found the image through a service such as Google Images, be sure to cite the actual web page the image was taken from and NOT the Google Images link.