1) What photo editing software do you have available?
2) If you don't have photo editing software do you attend a school where you might be able to gain access to such photo editing software?
3) Why didn't you level the horizons when you took the photos? Sorry, but I had to ask that.
Whatever software you use, one thing you'll also probably have to do after leveling the horizon is to crop the image to clip off the resulting areas of the image that have nothing in them. Either that, or with Photoshop you can clone in those areas with the rubber stamp tool.
if your into photography get some editing software. any photoshop, or the photoshop elements programs are awesome, and alot of fun to toy with... BUT for minor adjustments (like straightening a horizon) pretty much anything will suffice. you may be able to download something for free, but for stuff like that I use a microsoft program that is very simple, and levels by percentage. sometimes that one can't get it perfect tho, then I go to photoshop. I recently bought PSE6 for around 80$, and that came with all you need for minor photo adjusments and straightens manually, not by percentage. the photoshop elements programs are simple and don't come with all the tools, but for basic photo touchup, that's what I recommend. If you plan on digital art steer away from the elements programs and lay down the money and get dreamweaver... I don't have it but have used it and its sick.