Images not loading
When you don't see the preview of a file that you just uploaded, then it is likely that the server ran out of memory when it tried to generate the preview image. You can either manually resize the image before uploading so that the server can handle it, or increase the memory limit in the Designs configuration.
Long loading times
Preview images are not generated directly when uploading new files. Instead, they are generated on-demand, when someone wants to see this particular image. This means that, for the first time (and only this time) an image is requested to be shown, the preview file is generated which may take a little bit. This happens only once per image, or until the image cache is cleared.