I want to be able to insert a picture on to various types of backdrops/backgrounds. Like a green wall, which software can I use that is intuitive?
I think gimp is the best solution for you.
There are several applications that do this.
Also there are a few online images editors as well.
Here is the most robus application
use photoshop or gimp and manny other but PHotoshop is the best.........!!!
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The amount of different graphics software that will allow you to do this is massive. Which graphics software are you comfortable with? I mean this is even possible with MS Word.
How to insert an image on a background with GIMP
with paint you want to use star lasso tool, to select around object, copy, then paste in new window on blank canvas, edit with white brush to clean edges, , then select again with star lasso tool with the transparency option on and transparency option on new background image, then paste, this will only paste image with out white square
Photoshop would be great. if you are looking for free alternatives then there is gimp, paint.net, pixlr, etc.
You can use the free software PAINT.Net as it supports layers. You can get it from http://www.getpaint.net/index.html
Paint.net id good for the beginner.
Another free software would be GIMP http://www.gimp.org/ which is slightly more powerful
GIMP is a little more complex but there are numerous tutorials available on the net including Youtube
Commercial products abound including Adobe Photoshop or the slightly less powerful and more beginner friendly Photoshop elements