I (as recently arrived admin) enabled image upload re-size by setting ACP to 1280 x 900 pixels. (was formerly 0x0 or no resize).
Upload limit is set to 200KB if no resize occurs.
What I and forum members are finding is that re-size does not always occur and if the native size of the image is greater that 200KB then we get an attach error (yellow triangle)
The issue is browser dependent and device dependent.
At one stage using Safari browser Version 12.1.2 on Mac OSX 10.14.46 no issue with photos of 6MB down. Now (a few days later) and all image attachments get the yellow triangle that when clicked states that images must be 200KB or lower. Chrome browser Version 76.0.3809.100 on the same operating system has no issue uploading a 6MB image - the image being dropped in size to approx 223KB.
Windows 10 pro offers the same inconsistency.
Mobile devices also provide very random results. My iPhone 7 plus seems limited at 2.2MB- nay higher and unable to load. Android 6.1 running on old Nexus 7 tablet has no issue with 6MB files.
I suspect a server side issue - resource issue - but the issue seems to be independent of server load.