Events are worth 5 EP not 10 EP (10 EP is used for required assignments only). So if you did an event drawing, you'd get 5 more EP than usual.
To better explain, if you drew a fully colored event picture with 6 felines, one of them wouldn't be counted for the extra +2 you get for a picture (Consider that feline gets a +3 since that's what you get for a fully colored picture) and the other 5 would have a +2 modifier (which equals 10 extra EP for those five characters) So you would get the 3+5+10 which equals 18 EP.
If you want more EP, it would be better to submit a picture for each of your characters since they would all be +8 EP (5 for the Event and 3 for it being fully colored)
I hope that makes sense!