The images you are viewing have been cleaned to remove most of the cosmic rays, but the cleaning routines are far from perfect. Frame 1 is a single 30 minute exposure of WF4 before cosmic rays have been removed with sophisticated computer programs. In Frame 2, two consecutive 30-minute exposures were compared to remove cosmic rays. A constant background level has also been subtracted. The contrast has been adjusted to most nearly agree with Frame 1.
Most streaks are oblique cosmic ray hits.
You can see that cosmic rays are a major issue in long exposures. No computer program can perfectly remove all the cosmic rays. Cosmic ray hits are random. It is extremely unlikely that a cosmic ray will hit the exact same location on the chip in two or more of the six epochs you will be blinking. A cosmic ray will appear as a blip in only one image.