I'd love to know the pixel dimensions of the original custom image.

If I understand correctly, a custom image was uploaded as project profile image, but not rendered correctly and the mystery ghost got rendered instead.

Are there project picture criteria

I'd assume that the 1 ∶ <4 ratio also applies here, per T290959#9791029.

we need to document on the upload dialog?

Maybe we could expand the ->setCaption($supported_formats_message) in src/applications/project/controller/PhabricatorProjectEditPictureController.php, preferably in upstream:

Original image dimensions were 4672 × 5941, 3.8 MB (https://phabricator.wikimedia.org/F58028490). I then trimmed it down to be more square, resulting in 4667x4763, 4.3 MB (https://phabricator.wikimedia.org/F58028494). Both of those successfully uploaded, but displayed the mystery ghost.

The final image that worked was 400x400, 91 KB (https://phabricator.wikimedia.org/F58134690).

It seems like the dimensions of the first images were within the ratio -- was the file size too big, even though it allowed the upload?

In T383083#10434158, @HCoplin-WMF wrote:

Original image dimensions were 4672 × 5941, 3.8 MB (https://phabricator.wikimedia.org/F58028490). I then trimmed it down to be more square, resulting in 4667x4763, 4.3 MB (https://phabricator.wikimedia.org/F58028494). Both of those successfully uploaded, but displayed the mystery ghost.

The final image that worked was 400x400, 91 KB (https://phabricator.wikimedia.org/F58134690).

It seems like the dimensions of the first images were within the ratio -- was the file size too big, even though it allowed the upload?

oh! There used to be a 2 MB file size limit here, maybe that's what we're running into (rather than a dimension thing). I wonder if that's available in the EditPictureController(?)

I can reproduce locally, it's definitely the dimensions.
It is not conflicting with Phab's storage.mysql-engine.max-size setting (which, uhm, differs between production and GitLab) and it's not related to file size and it's not strictly related to 1:1 dimensions ratio. 4000px × 4000px works. 5000px × 5000px does not work. 4000px × 5000px doesn't work.
Let's see if I'll manage to locate some related code somewhere...

It's $max_pixels = (4096 * 4096); in https://we.phorge.it/source/phorge/browse/master/src/applications/files/transform/PhabricatorFileImageTransform.php$337 which means max 16777216 pixels.