Present simple works best for image description:

An image is in a "frozen" state in which the event is sort of always happening and not just happening right now.

To say it another way, this is a description of what we see, not what is happening.

The tone of the present simple form is more distant and appropriate for this purpose. The use of the present continuous form would sound like someone was telling us that Lord Voldemort is murdering Lily Potter right now.

for some other ideas:

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/simcon.htm