When you get on the reply page, scroll down to 'manage attachments', click and browse for your picture. A picture can't be more than 100 KB so you may have to re-size it.
If he is an indoor cat, I'm betting he hasn't gone far; have you been calling for him? Early morning and after dark are the best times, when there are fewer people around and he can hear you.
How long has he been gone?
Here are some websites you can post on:
You could also fax his picture to vets in your area and put up flyers. I wouldn't mention the amount of the reward, there are some crafty people out there.