For those wondering why photos taken from a phone might be rotated incorrectly when posting:
CoSo strips all EXIF data while uploading (for your safety and privacy). Among other things, that data may include rotation info, and wiping that out may cause your photos to un-rotate. This is a known bug, on the list to be addressed down the line.
To correct in the short term, make a small edit to your photo of any type, save that copy, and upload again.