Most photo editing, such as changing the tonality, colour and contrast of an image, is fairly easy but it's when you want to put things in or take things out that it starts to become complicated, and this is why programs like Gimp and Photoshop were invented. And because it's complicated, at least to start with, there can be a steep learning curve.
The jobs you want to do sound simple when you put them in words, but take a lot of processing to carry them out. For example, to put a blank background on a portrait photo: what is a blank background? Is it black, white, or another colour? Or would it be better to replace the background with another, less complicated one? In any case, you'll need to tell the program what you want to keep, by selecting it, then either erase the background or replace it or fill it with a colour. And if you try to 'blank out' gravestones, you'll obviously need to replace them with something or leave holes in the picture.
So you'd need to get to grips with selections, layers, and masks. And you'd need Photoshop CS, Photoshop Elements, Gimp, or something very similar. CS costs at least £280 (from one cheap site - usually nearer £500), whereas Elements is currently £50 from Amazon, and Gimp is free.
My recommendation is that you check out these tutorial sites and also search for "Gimp" on YouTube:
http://www.gimpguru.org/Tutorials/
http://meetthegimp.org/
http://vntutor.blogspot.com/2007/08/10-gimp-video-lessons-online.html
http://www.unixmen.com/linux-tutorials/1583-30-great-tutorials-for-gimp
http://www.scottphotographics.com/40-best-gimp-tutorials-of-2010/
http://gug.sunsite.dk/?page=tutorials
You might also like to check out the online book,
Grokking the Gimp, though it's for an old version, so you'd have to search the newer interface for the features you want.
But if you decide at any point to get more serious about photography, I'd say get Photoshop Elements while it's cheap. There are far more tutorials, many more users, and much more support.