Looks like this is a known bug and it still has not been fixed even 4 years after being reported:
https://github.com/PrestaShop/PrestaShop/issues/15183
If anyone else is having same problem even on P.S. 8.1, and if you are OK with modifying core files, open file src\Adapter\Image\ProductImageFileValidator.php and comment out the following lines:
if ($maxUploadSizeBytes->isGreaterThanZero() && $size->isGreaterThan($maxUploadSizeBytes)) {
throw UploadedImageSizeException::build((int) (string) $maxUploadSizeBytes);
}
This should allow you to upload files larger than 2MB (provided that you have php settings like upload_max_filesize larger than 2MB)