Meh
While your effort is admirable, it seems like there's a ton of stuff missing from this to make it overly interesting, and I can spot some parts where there are discolored pixels still in the image. It's good for Paint, but really if you wanted to make something cool you probably shouldn't be using paint at all...
The buildings look okay, and to be honest the flames look great. The smoke cloud is legit, and the burning debris is pretty convincing too. The problem I see is the lack of space usage. Most of the image is a blank green field, which is just an odd setting for the tall skyscrapers to be in. I could see if there were roads and people and cars, but it looks like some buildings in the middle of nowhere.
The sun was a nice touch, but the rest of the sky looks rather barren. I think some clouds would have been nice, or maybe some more smoke from the explosion. It's also a bit confusing what is causing all the chaos. There's a huge explosion to the right of the buildings, but I'm not really sure if that was a bomb or a fallen building. I don't see any planes that could have dropped the bombs, or any roads for a terrorist to have reached the city. It could obviously use some more art to give it a better theme in my opinion.
Overall what you have here looks good so far, but seems to be lacking a lot to make a complete picture of a nuclear holocaust. Keep at it though.
-RRC-