A file can be well under 1.5MB and still be bigger than 2MP (megapixels). A full-screen 1920x1080 picture is slightly over two megapixels, but it can easily be under 300KB in file size if compressed properly.
In any case, that message is coming from the software you're using, not from the server. You would get a different error message if the upload actually failed due to file size. The error you're receiving is telling you that your software is rejecting the image.