What is an extremely lightweight way to detect change in an image?

I'm trying to find the most lightweight method to detect "adequate" change in an image. I realize that's a terrible constraint, so for the sake of this question I'll break it into two different qualifiers: substantial change, or 80% pixels changed, and motion-changed, or 10% pixels changed. These images are captured via timelapse mode on a raspberry pi camera, but for the sake of the question, _the files could be from any basically-static view point_. I've found that I can use filesize to observe change of substantial amounts:

user@host-bb:/tmp$ ls -la investigating/
total 134604
drwxr-xr-x 2 user group    4096 Oct 20 00:05 .
drwxr-xr-x 5 user group    4096 Oct 20 00:04 ..
-rw-r--r-- 1 user group 4580181 Oct 20 00:05 image1018223300.jpg
-rw-r--r-- 1 user group 4573449 Oct 20 00:05 image1018223301.jpg
-rw-r--r-- 1 user group 4573024 Oct 20 00:05 image1018223304.jpg
...snip...
-rw-r--r-- 1 user group 4455867 Oct 20 00:05 image1018223320.jpg
-rw-r--r-- 1 user group 4447607 Oct 20 00:05 image1018223321.jpg
-rw-r--r-- 1 user group 4434839 Oct 20 00:05 image1018223323.jpg
-rw-r--r-- 1 user group 4416942 Oct 20 00:05 image1018223324.jpg
-rw-r--r-- 1 user group 1769008 Oct 20 00:05 image1018223325.jpg
-rw-r--r-- 1 user group 1702851 Oct 20 00:05 image1018223326.jpg
-rw-r--r-- 1 user group 1639306 Oct 20 00:05 image1018223327.jpg
-rw-r--r-- 1 user group 1788435 Oct 20 00:05 image1018223328.jpg
-rw-r--r-- 1 user group 1908061 Oct 20 00:05 image1018223329.jpg
-rw-r--r-- 1 user group 1981029 Oct 20 00:05 image1018223330.jpg
-rw-r--r-- 1 user group 2102390 Oct 20 00:05 image1018223331.jpg
-rw-r--r-- 1 user group 2167734 Oct 20 00:05 image1018223333.jpg
-rw-r--r-- 1 user group 2243082 Oct 20 00:05 image1018223334.jpg
...snip...
-rw-r--r-- 1 user group 2640732 Oct 20 00:05 image1018223342.jpg
-rw-r--r-- 1 user group 2730206 Oct 20 00:05 image1018223343.jpg
-rw-r--r-- 1 user group 2751966 Oct 20 00:05 image1018223344.jpg
-rw-r--r-- 1 user group  315875 Oct 20 00:05 image1018223345.jpg
-rw-r--r-- 1 user group  302476 Oct 20 00:05 image1018223352.jpg
...snip...
-rw-r--r-- 1 user group  285965 Oct 20 00:05 image1018223358.jpg
-rw-r--r-- 1 user group  289271 Oct 20 00:05 image1018223359.jpg
user@host-bb:/tmp$

Even between somewhat non-major changes in lighting (such as occurred between images *28.jpg and *29.jpg) can produce a difference in filesize. I realize this is basically binning, and it could be the solution I'm looking for. This takes care of the "substantial change" (80% of pixels changed). The question remains though: Short of Imagemagick, Python script, or other image processing software, is there a lightweight way to detect change in an image that is closer to 10% pixels changed? If there is no non-image-processing software option, what would the most light-weight option be? Writing my own processing in a low-level language? A Python script? I'm coming from a Python background and am trying to do simple change detection on a raspberry pi zero w that can then trigger an event (file upload) if there is a change. Bandwidth is too small to send all the images, and the pi zero w is not a heavy hitter when it comes to processing power.