I will try to create a log to find the problem. Meanwhile i see that not all pictures are invisible.
Some of them i can view and download, the invisible i cannot download on my phone.
So i download one invisible and one viewable picture on my linux system to see the different.
Strangewise 
both pictures have the same format except the name suffix …
Here the output on my linux …
gabs@s2:~/Downloads $ file IMG_6225.jpg
IMG_6225.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 1600x1200, components 3
gabs@s2:~/Downloads $ file signal-2023-08-19-072056.jpeg
signal-2023-08-19-072056.jpeg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 2048x873, components 3
The first one is visible the second not …