Server host is running ModSecurity
If when drag and dropping an image to add to your page, you receive a 403 error code, this most likely means your server host has ModSecurity enabled on your server.
You should be able to simply ask your hosting provider to disable ModSecurity because it's preventing your CMS from adding images.
Adding images before they have uploaded
When you drag and drop an image into the Go image upload area, wait until the thumbnail appears, this means the image has fully uploaded to your server, and is ready to be added to your page.
Images with large filesizes, naturally will take longer to upload. You can increase the upload speed with either lowering your image filesizes or upgrading to a quality web host with faster upload speeds.