Ok, what you see is a few dust spots at the bottom of your sensor (although in the image on top...mirror...) This is normal if you change a lens and you are not very careful how you go about it. Remember that dust is floating in the air all around us, we just don't always see it. You might think the house is clean, well...think again. I always try do change my lens where dust should be minimal, and the cleanest room in the house is usually a bathroom. Close the door to prevent any dust swirling around as a result of a draught. Clean the lens you are going to fit with a rocket blower, especially around the mount that is going to be fixed to the lens you notice the red line-up dot on the lens... place the lens with the mount facing up and the red dot in about the 2 o'clock position. Hold your camera facing down, and remove the lens you have on it, put it down and while keeping the camera facing down, move it over the lens you want to put on. The red dot of the lens should more or less line up with the red dot on the camera. Pick the lens up and engage it with the camera by feel, without turning the camera...at first it seems difficult, but with a bit of practice it comes natural. The whole process is to keep the area that is open on the camera, open for the least possible time, thus preventing dust from entering...
Regarding the dust spots you have now, a rocket blower should fix that(google) Read the manual (p261) to see how to do it, just make sure that your battery are properly charged when you do it..
Dust is not only a Pentax problem, but of any DSLR, where you expose the inside of the camera when changing lenses.