There are websites out there such as gimp and picnik or picasa that are similar to photoshop, but not nearly as in depth.
If you are doing something simples, like slightly changing the colours, adding words etc. those programs are fine.
If you want the real photoshop, that does more than these, you buy it.
It's not that expensive, and it's better in the long run. The newest photoshop, you cannot find a free or stolen version of, it just doesn't work.