Hair on the sensor.
A squeeze bulb blower is as cheap as chips and you should have one anyway. With the mirror locked up as per the instructions above, remove the lens, point the open camera down so the loosened dust will fall out, and give it a couple dozen vigorous squeezes. Remount the lens and survey the effect by taking a few more pictures of a blue sky. Based on the photos that should be enough.
You can of course repeat the blower if necessary.
If stubborn dust remains I personally feel a sensor brush is a very reasonable and conservative next step. I really would hate to go through the cost and inconvenience of mailing my camera to Canon for a sensor cleaning if I could easily avoid it, especially if my camera body was a $300 - $400 model like a T5 and not a $7,000.00 model like a 1dx2 or something, in which case my answer might be different. Just be careful and brush ONLY the sensor, not anything else in there, or you might pick up oil or other contaminants and smear it on the sensor.
That should almost certainly more than suffice for the kind of loose dust you seem to have. If the brush is not enough, then this might be the point where you decide discretion is the better part of valor and you give in to the professional cleaning. If you choose to plunge ahead, I suggest a lot of YouTube videos on sensor cleaning. I shy away from wet cleanings myself, but the tacky gel stick type cleaners I use are super super easy and I would not hesitate to buy and use one. Look up Sensor Gel Stick.
Scott
Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites
Why do so many people say "FER-tographer"? Do they take "fertographs"?
