Log in to Koken, go to Settings, then Image publishing and click Clear image cache. However, on a resource limited server clearing the entire image cache may fix one issue but introduce others. To fix an individual pixilated image, find the pixilated images(s) via the web browser. Then log into Koken and find the exact image(s) in the Library. Rotate the image and then rotate it back. This will force the server to re-render that specific image and the pixelation is gone. Also upgrade PHP to v7, it is significantly faster.
Let me try to explain what was happening. The issues you describe are due to your host’s resource limitations and may happen after a period of activity; the actual cause is on the hosting side.
Shared hosting accounts are exactly that; many websites share a server. Hosting companies place restrictions on the amount of server resources available to any one site to minimize the chance a resource intensive site will slow down every other site.
A key part of Koken involves image generation and caching. Koken creates images by using the image processing software installed on your web server. This process takes your uploaded images and generates smaller versions for use inside Koken and on your public web site. A variety of image sizes are generated for optimum load times across all screen sizes (phones, tablets, desktops, etc). Once smaller versions of images have been cached, those cached versions are re-used for optimum speed and performance. A cached copy of an image is only generated again if you replace its original file with a new version.
In all, Koken has the potential to create 28 different caches for each image you add to your library. That’s a lot of images, and more than most shared web hosts can handle being requested all at once. To workaround that limitation, Koken creates most cached images on-demand as needed to avoid overloading your web server. As the images are generated on demand (when the page is visited) it will take time until those images are created and cached again.
More info here