I agree. Next time, it would help a lot if you "store the original" image in your post with EXIF intact here so we can view it enlarged and see the shot details, camera model, etc.
But from what I can see (white spot near the bottom edge and left of center)... most likely it's on the sensor. A proper cleaning should solve it. Even with self-cleaning sensors, oil and dust occasionally end up requiring a cleaning. The reason you don't see it in some images is because it will be more obvious when using smaller apertures, more blurred and less noticeable with larger lens apertures. It also probably "hides" in image detail in some cases.
It's definitely not the lens, since it's occurring with different lenses (plus, to be that apparent in images anything in or on the lens would have to be very large and quite obvious to the naked eye).
I've never seen a dead pixel that looks like that. However, depending upon post processing, maybe sharpening or noise reduction or other treatments to the image are changing the appearance of a dead pixel.
Dead pixels are not uncommon. With literally millions of individual pixel sites on a sensor, is it any surprise that one or two might fail? Most cameras have some procedure for "mapping around" dead pixels, to solve the problem. Since we don't know what camera is being used, we can't really say if that's possible with yours.
The best info about sensor cleaning can be found at:
www.cleaningdigitalcameras.comThat website is maintained by professional camera repair techs... one of whom (Larry) literally "wrote the book" (actually many of them) on camera repair. The other tech (Curt) owns
www.micro-tools.com, which is a major supplier of tools and supplies to the camera repair industry. They have done thousands of cleaning themselves, stock and sell all types of sensor cleaning devices, and give unbiased feedback and advice about all the different methods.
If a camera's sensor has never been cleaned, it almost always will need a "wet cleaning", because it will usually have oil on it (from the shutter or other mechanisms). Any "dry" cleaning method will only smear the oil, contaminate the cleaning device and make matters worse. If at all uncertain about doing safe cleaning yourself or unwilling to invest in the approx. $100 to $150 worth of supplies and tools needed to do so properly, you can probably find someone locally who offers sensor cleaning service at a reasonable cost. Might even do it while you wait (just make sure your camera's battery is fully charged, in either case).