As far as I see, this is an image specific issue. Again. This is something that cannot be debugged. Image format, image size, image resolution, phone model and lots of other things may have some affections.
Also, it doesn't matter what is your server RAM size or how powerful is it. There are limits for image processing libraries in PHP and the limits are not GBs, they are small. Before the server, your browser should create a thumbnail from that big image. In most cases, this is a browser issue. Browsers cannot create Base64 based image content and display on preview section.
We can check one example if you provide the image, and full information of the browser and device you use to attach it. Thus, we could simulate the same case and provide more info.