simple lines (though it could be a bit tough) could be done in a simple program such as MSpaint using the paint brush tool. But if you want to actually alter a photo to actually see what a tree could look like, a more advanced program is best. I use Adobe Photoshop CS2 (I believe its up to CS4 now?) for editing, the only thing about photoshop is that it can take some time to learn. I taught myself, so it can be done, it'll just take some time. Best thing is to look up tutorials online and just start playing with images. The good thing is, images are not like trees, so you can always click the undo botton (now if there was just one for real life)
As I just don't have the hundreds of dollars for Photoshop I use Gimp 2.6. It is free, it works on any operating system and just as powerful (better in some areas not as good in others) as Photoshop albeit there is a learning curve to the commands.
You can do a lot with Photoshop, or even Photoshop Elements--but the software is not really the key.
There are 2 things you have to know: 1) You have to have a very clear idea of what direction you want to take your tree in, and 2) You have to know the tools that Photoshop has, and be able to use them with some skill, in order to make the right decisions, graphically, to achieve your aim.
Most virtuals I see don't display enough basic familiarity with Photoshop to be able to express the user's intentions clearly.
If you want most of the functionality of Photoshop without the price, try using the Phoenix image editor at http://aviary.com
They have several other image and audio editing programs that are all free, web based, and named after birds. You have to create an account to save whatever images you create. You can use an existing twitter, facebook, google, or yahoo account to log in also.