Barrel Distortion (also called Pincushion Distortion) -- there is a slight convex curve that appears on straight lines in images -- it is caused of a camera's convex lens shape.

Photos taken with lenses that have zoom capabilities suffer from barrel distortion. Because barrel distortion makes the outer areas of your image rounded, it's most apparent when there's a straight edge in the image, near the frame. See the original Basic Forms image from the Interactive Art School's Lesson 1 in its original form just below

Now below this is the same image with a simulation of a lens caused barrel distortion (aka pin cushion distortion):

You can minimize or eliminate this effect by shooting from a greater distance (between your camera and the drawing  or object you are shooting. First below is photo simulation of shooting tightly on the subject (with very little space between the subject and the edge of the photo (the black line represents the edge of a drawing you are shooting and the red line the edge of the photo:

Now, my suggested shooting leaving much more space between the drawing your are shooting and the edge of you photo framing (the red dotted line is the  original framing above and the blue line my suggested framing (in other words step the camera back from the drawing:

More about camera distortion at:

http://en.wikipedia.org/wiki/Distortion_(optics)

The same framing issue (step your camera back from the drawing you are photographing) so that you don't cut off parts of the drawing. Most digital cameras have plenty of resolution so that the central area of the actual drawing will be well resolved for use in our online critiques

More details about convex lenses at:
http://en.wikipedia.org/wiki/Lens_(optics)