Found the problem... the forum software has an undocumented limit on the dimensions of an upload image, but I don’t know whether it’s a limit on the raw number of pixels (yours is 16,084,992) or on the height/width of the image.
If you scale the image down to 3000 × 4529 @ 240 ppi, you should be able to upload it as long as the file size is 3MB or less.
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gonZo