Yes, it is clearly dust
Try a rocket blower before anything else.
Follow this link:
Giottos Rocket Air Blower Review
They are not expensive and very effective against the type of dust (large particles) you have suffering from.
Read the camera manual how to lock up the mirror and open the shutter.
Keep the camera with its opening down (let gravity drop the removed dust out) and apply the blower.
Make sure not to touch the sensor with the blowers nozzle.
If you cannot remove this with a blower (what I doubt), you can either have it cleaned at your local camera shop or buy one of the many sensor swipe solutions.
Depending upon where you live, prices may differ. For instance my local camera shop does a cleaning job for less money than a good wet swipe set.
The K10D is a lot more sensitive to collecting dust than the models introduced after the K20D.
For what it is worth (it help somewhat), do you have the auto sensor cleaning done when the camera gets turned on?
If not, you should. See the camera's manual.
Good luck, Bert