That looks like a small hair fiber - nothing to worry about.
Shoot at a very high f-stop to test for dust (e.g. f/22 or even f/32). The tiny aperture opening causes any dust or fibers to cast a nicely defined shadow so you can easily see where the dust is located. At wide apertures, the shadow is not well-defined and you can miss noticing the problems.
Also, the image on the sensor is upside-down and backward. Dust visible in the upper-right section of your image means the dust fiber is actually located in the lower left section of the sensor.
Dust is a normal fact of owning and using a DSLR. When you remove the lens, dust can get in. At that moment the shutter is closed. But when you attach a lens and take a shot, the reflex mirror quickly snaps up (creating a whoosh of air inside the camera) and the shutter opens... offering the dust a breeze and a chance to re-settle somewhere else... such as on your sensor.
The good news is that this isn't a defect (your sensor isn't damaged).
As kvbarkley says... time to get a "rocket blower" (Giottos Rocket Blower is the most popular hand-squeezed air blower).
In a clean environment, remove the lens, point the camera at the floor and invoke a dust clean cycle.
Put the lens back on, put the camera in Av mode, dial in the highest aperture you have, point the camera at a plain white wall (don't even focus) or plain blue sky ... and take a photo. Inspect to see if you can still see the dust.
If so, remove the lens, turn on manual sensor cleaning mode (this locks the mirror up and opens the shutter so you can access it) and blow puffs of air in the area of the dust (remember the images are upside down & backward) while BEING CAREFUL not to let the tip of the blower hit the mirror, sensor surface, etc. DO NOT use cans of compressed air (products like "Dust Off" spray a propellant and leaves a foggy residue and doesn't evaporate so that results in having to manually clean the sensor with a wet-cleaning product.)
If that doesn't work.... you may need a pristinely clean brush (think "paint brush"... but it must be pristinely clean, don't use a used brush and don't run the brush across your hand to let it collect skin oils, etc.). There are special sensor cleaning brushes that have a grounding wire so it will discharge any static electricity (in case the dust is persistently clinging due to static build-up).
If that doesn't work... then you end up having to "wet clean" the sensor. The popular product is called "Eclipse" cleaning solution (by Photographic Solutions, Inc) and you need the swabs ("Sensor Swabs"). Lots of YouTube videos demonstrate this... but you put just a few (and I do mean "few" as in maybe 3) drops of cleaning solution on the swab and wipe the sensor once, then toss the swab in the trash bin (never re-use a swab... don't want to risk scratching the sensor by dragging a bit of sand, etc. around).
The cleaning solution is nearly pure methanol. It evaporates quickly and leaves virtually no residue.
While I do caution that you want to be gentle to avoid scratching the sensor, the sensor (it's really the filter in front of the sensor) is "glass" and "glass" is "hard". It wont scratch easily ... but don't push it.
Be gentle, read and follow directions... and it's pretty easy to do.
You can optionally invest in something called a "sensor scope". It's basically a special magnifying loupe designed to fit on the lens flange with a light on it so you can inspect the sensor for dust.
If you use your camera regularly, I promise this wont be the last time you get a bit of dust in the camera. You can take the camera in to have it cleaned or even send it out... but you may as well learn to do this task yourself since it's really not very difficult and it'll be a lot cheaper to do it yourself.
Tim Campbell
5D III, 5D IV, 60Da
