The size of installation package of Layer Slider is about 6M. Sometimes on some servers, the maximum uploaded file size can be only 2M. So you need to increase the max_upload_size PHP variable to a larger value, like 10M in the PHP ini file, or if you can't make it, just ask your hosting provider. If this modification won't solve the problem, you also need to check the post_max_size PHP variable, and also increase it for a higher value.