Encode for the destination, not by habit
Every extra bit depth, color channel, and alpha byte is weight the destination may never render. Print-bound JPEGs belong in CMYK before they leave your hands, and a scanned document that lives in grayscale should never carry three color channels to say one gray. The first tuning decision is always about where the image lands, because a correct color mode chosen early saves re-encoding the same file three times later.
PNG depth is a content decision
PNG weight is decided mostly by depth and palette choices. Flat art, UI screenshots, and icons sit happily in an indexed palette or 8-bit depth and shrink dramatically. Opaque photographs need 24-bit RGB, and 32-bit RGBA is justified only when the layout really composites the image over varying backgrounds — decorative alpha is ballast in every channel. High-precision grayscale work is the one case for 16-bit, and it should stay rare.
Loading behavior is a user-facing feature
Progressive JPEG and interlaced PNG change how the page feels on a slow connection, letting content appear early instead of top-down. That benefit has a price in pipeline compatibility — some older editors and batch processors mishandle progressive scans — which is exactly why the baseline conversion exists. Choose loading behavior per destination, write the choice down, and keep the fallback one conversion away.
Quality steps and the closing compression pass
Quality is not one global slider. A screenshot and a photograph tolerate different JPEG quality steps, and WebP splits into lossless, near-lossless, and lossy modes that map cleanly onto content types — crisp edges versus gradient-heavy scenes. Set the quality step per content type, then let format-native optimization squeeze the final bytes as a closing pass, rather than re-rolling encodes at random settings hoping for smaller output.
WebP delivery and the archive that remains
When the site standardizes on WebP, resize stays inside the format and alpha is stripped where the layout allows, with whole sets converted in one batch. TIFF masters, meanwhile, are not web assets — compress them for storage with an explicit method so the archive stays both smaller and reproducible. The finished state is a delivery set where every image's depth, mode, loading behavior, and quality step can be read back from your notes, not guessed from the file.