Would have been helpful if you provided the filename. I didn't even know that you could save a screenshot from the Speed Test app. Just gave it a try and see that the name is "DiskSpeedTest.png". Here's a test of uploading/inserting...
The image in my post is directly from the screenshot function in Blackmagic Disk Speed Test (version 3.2) and it has the original name, "DiskSpeedTest.png". It was created on my 2018 Mini running Catalina.
Same thing with a screen shot. 1st try was without the space, 2nd with the space (Screen Shot 2022-07-04 at 9.27.05 PM .png). I do have some other screen capture utilities installed, such as Snagit, but I wouldn't think it would make a difference.
Tried the insert again after removing the space (Screen Shot 2022-07-04 at 9.27.05 PM.png) and it worked. The upload results screen shot below from the 1st test where it didn't work uploaded however with no problems. Took another screenshot a 3rd time without changing the filename (Screen Shot 2022-07-04 at 9.32.04 PM.png). Worked. Confusing.
Never happened before migrating to a 12.4 Studio. It started happening after I had to wipe the boot disk and do a complete TM restore.
Maybe it doesn't like the sounds of all the cherry bombs being set off right now for the 4th.