Criando um restserver com codeigniter - upload de imagem

welcome_message.php

<div class="modal fade" id="formUsuarioModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
		<div class="modal-dialog" role="document">
			<div class="modal-content">
				<div class="modal-header">
					<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
					<h4 class="modal-title" id="myModalLabel">Novo Usuário</h4>
				</div>
				<div class="modal-body">
					<form class="form-horizontal" method="POST" action="<?=base_url('api/usuarios')?>" id="formUsuario" enctype="multipart/form-data">
						<div class="form-group">
							<label class="col-md-3 control-label" for="nome">Nome</label>
							<div class="col-md-9">
								<input id="nome" name="nome" placeholder="Seu nome" class="form-control input-md" type="text">
							</div>
						</div>

						<div class="form-group">
							<label class="col-md-3 control-label" for="email">Email</label>
							<div class="col-md-9">
								<input id="email" name="email" placeholder="Seu email" class="form-control input-md" type="email">
							</div>
						</div>

						<div class="form-group">
							<label class="col-md-3 control-label" for="senha">Senha</label>
							<div class="col-md-9">
								<input id="senha" name="senha" placeholder="senha" class="form-control input-md" type="password">
							</div>
						</div>

						<div class="form-group">
							<label class="col-md-3 control-label" for="biografia">Biografia</label>
							<div class="col-md-9">
								<textarea class="form-control" id="biografia" name="biografia" placeholder="Fale um pouco sobre você"></textarea>
							</div>
						</div>

						<div class="form-group" id="foto-input">
							<label class="col-md-3 control-label" for="foto">Foto</label>
							<div class="col-md-9">
								<input id="foto" name="avatar" placeholder="Foto de perfil" type="file">
							</div>
						</div>
					</form>

				</div>
				<div class="modal-footer">
					<button type="button" class="btn btn-danger pull-left hidden" data-dismiss="modal" id="btn-remover-usuario" >Remover</button>
					<button type="button" class="btn btn-default" data-dismiss="modal">Cancelar</button>
					<button type="button" class="btn btn-primary" id="btn-submit-form">Salvar</button>
				</div>
			</div>
		</div>
	</div>