Every image upload is given 60 seconds to finish, and the attempt is abandoned when it runs out. That limit is on the clock, not on the file size, which is why a 19 MB image can fail even though the limit is 20 MB: it is inside the size rule and outside the time rule.
Nothing is saved when an upload times out. The transfer is cut off before the server finishes reading it, so you have not half-added an image and you have nothing to clean up. You will see Failed to upload, followed by the file name, and a suggestion to try again. On the cover image the message is shorter: Upload timed out.
Work out whether time is really your problem
Sixty seconds for a 20 MB file means the upload has to hold roughly 2.7 megabits per second the whole way. Office and campus connections are usually far quicker than that on download and much slower on upload, which is why the same file can go up at home and fail at a studio.
Three signs point at the clock rather than the file:
- The progress bar moves normally and stops near the end.
- The failure arrives about a minute after you started, every time.
- Smaller files from the same folder upload without trouble.
If instead the file is rejected instantly, it is a size or format rule and not a timeout. Why a file was refused covers those.
What to do about it
- Export smaller. Images wider or taller than 4000 pixels are shrunk on arrival anyway, so exporting at 4000 pixels on the long edge costs you nothing in what visitors see and cuts the upload time sharply. See Large images are resized when you upload them.
- Save as JPEG rather than PNG for photographs and renders. A PNG of the same picture is often several times larger.
- Upload fewer at a time. Images go up one after another, and each one gets its own 60 seconds, so a queue of ten does not shorten anyone's allowance. What it does do is give a slow connection ten chances to fail.
- Retry on a wired connection if one is available. Retrying on the same connection with the same file usually produces the same result.
Video is not on this clock
MP4 video does not go through the site at all. It is sent straight to storage, with no 60-second limit, which is how a 2 GB file can be uploaded at all. A video failing is a different problem: see What the video statuses mean.
Deleting media has its own, shorter limit of 30 seconds, after which you get Delete operation timed out. Retry the delete; the image is normally still there.