www.lmaowtf.com Image sharing website (Yeah I know It's not an original idea).

Hi, I'm new to site point. I really like responsive and resourceful community. That's why I'm asking for feedback for my website.

Website Background
I am a novice php programmer. I bought the domain (lmaowtf.com) so I decided to start a project that kind of matched the name. This is the first idea I came up with. It is my first fully functional php/mysql service. I was inspired by fatpita.net mostly, but I wanted to make a website where users add their own content like imgur.com or 9gag.com. My main focus (besides coding my first functional php/mysql website) was for it to be simple and flexible. Users click the image on the main page and get another random image. They can also browse all the images in the database, or browse by the users who uploaded pictures, or browse by tags attached to the picture. I think my favorite part is the tagcloud. I would have to have an enormous tagcloud with hundreds of tags that show which series are more popular and whatnot. Right now there are only three tags, but if there were more I'd dedicate a whole page to tags.

Please review the following:
I kind of started and finished this website in a hurry so I know the css and html is really messy and more clunky than it should be. However, I did try my best to make it a somewhat flexible design. I know there probably might be Internet Explorer issues.
  • Security. Because of my small knowledge of php/mysql, my main worry is that there might be security holes. If anyone can spot flaws in one of the pages that a user can take advantage of please tell me. I tried to prevent sql injection as much as possible but I might have missed something. My biggest fear is getting hacked. I know it might be highly possible right now.
  • The design. I wanted to try to keep it simple so the one picture stands out on the main page. I like the navigation bar but i'm open to suggestions. The tag cloud is really the main part of it (even though theres only 3 tags right now). I reallly don't know what needs work. If anyone has a problem with anything please tell me.
  • Adsense. I really struggled to think of good spots to put ads. I don't want to annoy users with big bulk ads and i dont want to distract from the focus picture. I put 1 ad on the bottom right (it randomly swaps out with the facebook like plugin). I put a second ad at the very bottom of the content <div>.

Basically, if anyone has suggestions whether it be with the design, the coding, or ANYTHING. I'm very very open to your opinions. I know an image sharing website like this isn't very original. I don't expect it to be competition for imgur or fatpita or 9gag. If it's possible for this website to someday drive even 100 people a day to it. That would make this a HUGE success in my eyes. Remember, this was just a random project I started about 2 weeks ago. This is the only website I've ever made that has a purpose for its users. Before I've only made portfolios. (like my portfolio website www.jeremypreed.com but you dont have to look at that.)

Thanks for any response! Sorry for making my post so long.