About

Promotions are image macros, as well as a common 4chan slang, used as a response toward a thread that got Not Safe For Work content in it while being in a board meant to have Safe For Work content only.
It basically means that the one who posted this NSFW (Not safe for work pun here) content will eventually get busted at work doing it, be fired and that someone else will take his job, hence the promotion as some kind of "wish".
That is to say, it's ironically used to say "Stop posting NSFW content here before it got deleted or that someone will gain a promotion thank to you."
The image macros are usually featuring someone who is triumphantly raising his arms and grinning in self-praise, with sometimes the "promotion" word underneath.

Meme context

Initially, On 4chan, boards are divided into the SFW ones and the NSFW ones, and each has a definite topic such as mangas, cosplay, movies, video games and so forth.
Contrary to /b/ where almost everything is allowed, there are more active moderators and more "enforced" rules on SFW boards.
Despite that fact, some users like to troll people by making threads with explicit content in it, while still having a topic in common with the board they're in, like picture of video game characters involved in sexual intercourse for the /v/ (video game) board, sexual intercourse featuring anime characters for the /a/ (anime) board and so forth:

Here is a 4chan archive example dating from 2008:

[ WARNING: 4chan's archive so NSFW content ahead ]

http://4chanarchive.org/brchive/dspl_thread.php5?thread_id=22821511&x=Left+4+dead+garys+mod

(There are numerous 4chan archived threads featuring explicit materials on boards they weren't meant for. These archives also featured in a more clearer way the use of the Promotions meme with sentences like "I just got promoted !" or "My salary just doubled !".
But they're way too explicit to be shown here, contrary to the archive link above.)

Here is the LurkMore wiki explanation on this:

Thus, whenever someone posts an NSFW picture, it was typical for a user to shout "I JUST GOT PROMOTED" in an ironic sense, meaning to imply that he actually just got fired for looking at porn at work, and that someone else would be promptly promoted to take his spot in the company. It originally started when Black_Knight (a 4chan user) flooded every NSFW thread with the post "I just got fired asshole," which only egged trolls on to post more NSFW images, much to the dismay of moderators and janitors. Anonymous later latched onto this idea and created the "Promotions" variation as explained above.

Origin

As pointed out by that first quote:

It originally started when Black_Knight (a 4chan user) flooded every NSFW thread with the post "I just got fired asshole," which only egged trolls on to post more NSFW images, much to the dismay of moderators and janitors.

An user named Black_Knight gave a first glimpse of what will further be the meme by repeatedly posting on NSFW threads from SFW boards that he got fired because of stumbling upon the disturbing content which, contrary to do anything to help improving the situation, only egged trolls to post more explicit stuff.
There is no traces to confirm that those events really took place as written however.

Then, as said by LurkMore wiki as well:

For the original "Fired" meme, the most ubiquitous is an image of a sullen, just-fired employee in a yellow shirt carrying out his belongings in a box. Once it morphed into the "Promotions" meme, one image that became popular was the character Phil Ken Sebben from a scene Harvey Birdman, Attorney at Law, in which he repeatedly yelled "Prooooooomotions!". Both of these, however, are now pretty much defunct and outdated, and are rarely if ever used or seen on the boards anymore.

There where two primary attempts at creating a meme from the initial catchphrase that didn't catch on.
After that, according to LurkMore once again, the meme really caught on 4chan's /v/ board around september 2006 when someone posted the picture of a middle-aged black male employee as a response with the "Promotion" word beside it.

There is no 4chan archive from that exact period, sadly.
The earliest mention of that picture in the Promotion meme comes from one of the initial photo's alteration, on a /b/ thread from May 2007. This thread was called "4chan pwnd Subeta":

[ Once again, 4chan's archive, NSFW content ]

http://4chanarchive.org/brchive/dspl_thread.php5?thread_id=26139544&x=4chan+pwnd+subeta#26141116

Alterations

From /v/, the "Promotions" picture spread to all the other 4chan's work safe boards and became the target of alterations.
At first, the initial photograph was modified while keeping the employee idea behind it but then, was related to the meme every picture of a person with the exact same pose.

Outside 4chan ?

This meme is very active within the 4chan's community, especially on work safe boards as one can imagine, but, because of the broad name given to it, it's difficult to say if it became used on other boards or websites.

So far, aside from the LurkMore article, the Macrochan entry and the ED's article, Urban Dictionary entries were made starting in 2007.

Furthermore, the meme seems to show up a little on DeviantArt, drown beneath many non-related pictures, as can be seen here:

• http://www.deviantart.com/#order=9&q=promotions
• http://www.deviantart.com/#order=9&q=got+promoted