One of the biggest technical differences between File Manager and Resources is the automatic image-optimization that’s built into Resources.
File Manager requires users to optimize photos before uploading them for best results on the web. Rather than saving images directly from a camera to the website, admins have to edit those images first - resizing them and saving them in a compressed, web-friendly format. Skipping this process may result in pages that load slowly, or that stall out trying to download enormous 5 MB raw photograph files. This in turn leads to a frustrating experience for users (and for admins, who must figure out why a page might be loading erratically and manually fix the problem images).
Resources fixes the problem of slow, aggravating page-loads by automating the process of resizing and compressing images. Rather than having an admin manually edit every single photo that’s uploaded (and possibly missing some!), Resources applies a single best-practice optimization process to each image. By letting Resources handle the technical work the same way every time, all of the images that appear on site pages are guaranteed to have consistent sizing and compression, which translates to faster, smoother page loads.
The responsive nature of Composer means that it automatically reformats pages based on the width of each user’s device screen. The same page will be laid out differently on a screen that’s 1200 pixels wide versus one that’s 750 pixels wide - page elements will stack up vertically on the smaller screen rather than spreading out horizontally. This same flexibility applies to the actual image files that are loaded on each device, as well - the smaller screen will download smaller-sized versions of the page’s image files, whereas a full size device will download larger-sized files. Resources automatically creates and compresses these differently-sized versions whenever an image is uploaded, so images do not need to be compressed in advance. In fact, uploading a pre-compressed image may give a very blurry end result after Resources applies its own compression.
The responsive alternate-size images that Resources creates do not count against a site’s disk space. As a result, the image compression algorithms in Resources are set to favor image quality over file size. The disk space allotments for each site are sized to try to ensure that most admins won’t have to worry about pre-scaling images before they’re uploaded (even using humongous 9 MB image files would mean you’d have to upload 2,000 images before you’d run out of disk space!)
The best practice for uploading media then is to upload un-optimized files, and allow Resources to handle the resizing and compression automatically.