Photoshop is used loosely a lot of times. Personally i use the real Adobe Photoshop CS5, but there alot of other image editing programs out there (ie gimp, coral paintshop pro, elements, etc) to edit pictures similar to the actual Photoshop. The Adobe Photoshop is a program you buy, but the aforementioned programs have a some similar tools and most of the time can accomplish similar task at a fraction of the price. Although nowadays just about any program can be had for free via the internet, but i won't get into that now.
Now as far a "How to Photoshop", that takes a lot, i mean A LOT of practice. There is so many different techniques that are involved in Photoshopping a vehicle (such as rim change, chop top, color change, lower, lift, render, tint, etc etc). There are several online sites that are dedicated to to Photoshopping vehicles, that are loaded with different tutorials and such. If you're wanting to get into it, I'd be more then happy to help you out! The reason some people can and others cant do it, is mostly because of the price of the programs, the difficulty for some, and because of the amount of practice it takes to become fluent with the program. It takes patients and alot of practice, but it is a very handy and useful tool to have!
Here is a link to Adobe Photoshop:
http://www.adobe.com/products/photoshop.html
Here's a link to a site that has a ton of good tutorials:
http://www.digimods.co.uk/tutorials/tutorials.htm