I'm sure there's an easier way to do it, but if you do a search on this forum, you will definitely find MANY different ways of reducing the pixels.
I use the Paint program in Windows. I pull up the picture and then go to the IMAGE tab. Then I click STRETCH/SKEW in that TAB and reduce the pixels to 25/25 or 50/50, depending on how big the original file is.
There are many other ways, as I said, if you do a SEARCH on this forum. There may be at least a dozen or more threads about this topic.