Photoshopping is an online sport involving the manipulation of images to create new works of great wonder and beauty (or, sometimes, complete crap). Most - but not all - of these works will employ a level of humour. The most effective (i.e. those that reach the most people) will contain some form of social, cultural or political commentary. Photoshopping is an extremely effective way of making an astute observation, or just making people laugh for the sheer hell of it.
This is a collection of some of my finer works. Please excuse me for being proud of them.
If you want to get into Photoshopping, you'll need Adobe Photoshop. You may wish to struggle along with lesser image editing tools, but Photoshop is the mutt's.
If you wish to share images via communities, the bare minimum you'll need is a place to keep them so they can be viewed remotely. Not all free hosting options allow this option, but B3ta points out a good option and how to use it here. If you have your own site and/or webspace, then you may as well keep your images in a dedicated location there.
The messageboard at B3ta (you pronounce it 'bee-tah', I choose 'bay-tah') is the best place to frequent if you wish to gain or maintain a creative edge (and my own B3ta profile can be seen here, if you're interested). The board features an endless variety of themes sparked by the moderators and the community itself. Things move fast, wild and rather random at B3ta; rather than simply dive in, try to watch the board for a few days first and do read the FAQ before posting, there's a good chap.
The discussion threads at Fark.com (particularly the Photoshop competitions) are great for honing your overall skill and - especially - speed. You'll want to watch a few competitions first, then join TotalFark so you learn about new competitions straight away. Be fast, be nasty, but play nice and you'll get by just fine.
The SomethingAwful site requires paid registration if you wish to play with the other children, but offers greater creative and technical challenges. SomethingAwful users pride themselves on style and substance, so a rough job aiming for a cheap laugh won't get you far here. Watch some of their featured threads for a sample of what this crowd can achieve.
Worth1000 is the most ordered option by far, and its users are well on par with those who mostly frequent SomethingAwful. The competitions at Worth1000 are great if you wish to take your time creating something wonderful, unbelievable, yet somehow completely convincing to the eye.