Вы можете использовать это для загрузки изображения
Future uploadImage(File imageFile)async{
var stream= new http.ByteStream(DelegatingStream.typed(imageFile.openRead()));
var length= await imageFile.length();
var uri = Uri.parse("Image upload url");
var request = new http.MultipartRequest("POST", uri);
var filename = "Your image name";
var multipartFile = new http.MultipartFile("image", stream, length, filename: basename(filename));
request.files.add(multipartFile);
var response = await request.send();
if(response.statusCode==200){
print("Image Uploaded");
}else{
print("Upload Failed");
}
}