As stated in ToS (Terms of Service) http://www.aeriagames.com/policy/terms
at point 7 (user conduct):
Terms of Service wrote:
You agree not to take any of the following actions:
(a) Upload, post, email, transmit or otherwise make available any Submission (as defined below) that is unlawful, harmful, threatening, abusive, harassing, tortuous, defamatory, vulgar, obscene, libelous, invasive of another's privacy, hateful, or racially, ethnically or otherwise objectionable;
These flashing pictures you are talking about can be harmful, so they are yet forbidden by the contract we all submitted when we created an account on Aeria.
As any other company that manage a public forum, "AGE does not pre-screen or monitor any Submissions" (another quote from ToS at point 6 - User Submissions), anyway they can remove these images, if they are made aware of the issue.
So what to do?
1) In first place try to made people aware of the issue, I'm sure most of the players that used these flashing images didn't think they could do any harm (this post is a good start, but I suggest you to make a similar post in your game section to let it be more visible).
2) If you see anyone using flashing image, send him/her a pm and ask to change/remove it
3) If he doesn't understand send a ticket to report him/her using this link http://www.aeriagames.com/contact/support
and choosing report behaviour as subcategory.
You can actually skip the 2nd point, it is up to you if you want to start nicely... Anyway don't be too reluctant to report the player: AGE usually doesn't ban for these reasons, just remove the inappropriate images.