You either have to load the images on start-up so you can get them instantly, but leading to higher memory usage.
Or load them from the files when needed, based on file size and the speed of the used hardware it will take some time to load.
So there is no real other way afaik.
One thing comes to mind is having a low res version (which won't use much memory) that you load on start up, so you can instantly display a low res version until the proper image is loaded via URL.