First thing to do is get a Giottos Rocket Blower and do a dry cleaning. This will get most of what's on there. I've only had to wet-clean the sensor on any of my cameras a total of twice in all my years of shooting.
If you do a wet cleaning like the one above, my best advice is to be conservative. It's easy to get too aggressive and scratch something by accident. So take your time, be careful, and breathe deeply. You know: measure twice, cut once.
Puikios nuotraukos, Erikai. Gražios kompozicijos, ir kaip jau aukščiau manęs asmuo minėjo, fainas post-production. Spalvos gražios, tik gal vietom biški dar su saturacija ir kontrastu pažaist reiktų, bet apart to puiki fotografija.
Got the Rocket Blower and the sensor seems to be clean now – at least I can't identify any grain of dust on it. But there still are some ugly spots in the pictures ... I'm afraid these come from inside the lens system ... Is there anything one can do about that?
Or is it possible there are still particles on the sensor, even if they seem to be invisible?
It can be very difficult to determine whether or not the sensor is clean, just by eyesight. Try taking some test shots to see how many spots there are on your pictures. You can do that by taking pictures of something with a white background, like a white wall or a piece of paper, just make sure it's a clean and even surface without holes or spots on it. I usually do it by shooting in aperture priority and at the minimum aperture possible (i.e. f/22 or higher). And I then set focus to manual and use the shortest focus distance. Make sure to fill the frame with whatever background you're using and then take a sample shot. To help determine whether it's the sensor or the lens that is dirty, you can try using to different lenses. If the spots remain in the same places even when using different lenses, then it's the sensor that is dirty.
Keep in mind that no matter how well you clean your sensor, there will most likely always be some visible spots when shooting at such a low aperture. When I have cleaned my sensor and I'm happy with the result, I usually take some shots that are closer to a real-life situation, using a more reasonable aperture. I almost never use smaller than f/12, so this is where I typically test it to see whether I can pick up any spots using that. Using the extreme end of the minimum aperture scale will almost always cause you to see some sort of spots, so that's why I do this.
But once I can't see any spots on the sensor within my normal aperture range, then I'm done cleaning the sensor.