That is called a "get" variable. It could be doing one of a couple (or more) things here.
's accepted answer to see what I mean.
Other times, what you will see is an ID, base64 encoded information, or other reference for the server that tells it what image you are fetching. This would be done in a case where the "../images/watch-wrapper-bg.png" request is rewritten on the server to run a PHP or ASP script (or the like) which reads the variable provided and then either alters a base image (like for game stats on forum signature images you might see if you ever wander into a gaming forum) or else does some other creation, customization, or even just basic file selection based on the variable provided and then serves up that specialized image for that request. So in this case the variable would actually control the image that you receive. If you're familiar with Captcha user verification that a lot of sites do then you've seen something like this in action. Play around with this text-to-image converter
for a more direct look at this type of use.
So, in general, it probably does "something." It might be a throwaway variable that the server ignores and is only there to keep your browser happy, or it might directly affect the response the server gives.