Resources includes features to automatically serve the best-sized images and videos for a particular user’s screen size, but there may still be occasions where a little bit of optimization is called for before you upload.
For the majority of your images and videos, no pre-upload optimization is required: the advanced compression algorithms built into Resources are as good or better at shrinking file sizes than other dedicated software and striking a balance between quality and size. For the largest files however (ultra-high-res photos and 4K videos, for example), even after Resources is through with it you may still end up with a file that's too big to download reliably.
In those cases, image-compression sites like Compressor.io can reduce the size of your file up to 90% while still maintaining high image quality, saving you both disk space and load times. Handbrake.fr offers a similar service for videos. Programs like Photoshop have an option for web optimization, as well. Taking advantage of these tools before you upload to Resources will help keep your files lean and quick, without compromising the experience for the end users.
In those cases where you want to make sure that your readers can view very-high-quality files, you can make them available for download and provide a link to the highest-quality version, but also keep a low-quality version on hand for display on site pages. This will keep download speeds low, making for happy visitors, while still making sure that those who are interested in your high-quality media are able to view it.