Drop a photo, get it at exactly 1500×500 pixels (3:1) — the X (Twitter) header size. Center cover-cropped, compressed, GPS/EXIF stripped. Nothing is uploaded; it all runs in your browser.
1500×500 is the header (banner) size X recommends — an unusual 3:1 strip that almost no photo comes out of a camera as. That's the real job on this page: the center cover-crop turns your photo into the strip, keeping the middle band and trimming top and bottom. The 1 MB preset budget is far below the platform cap (guides cite around 5 MB) and lands near-maximum JPEG quality at these pixels.
From a 4:3 phone photo, the 3:1 band keeps only the middle ~44% of the height. Sky-heavy landscapes, skylines and desks survive beautifully; faces usually don't, unless they're dead center. PixFit warns with both aspect ratios when it crops — if the wrong band survives, crop the photo roughly wide-and-short in your photos app first, then drop it here for the exact pixels.
Your profile picture overlaps the header's bottom-left corner on the profile page, and different screen widths shave a little off the header's edges — so keep anything important centered and away from the bottom-left. The re-encode strips the photo's EXIF and GPS as a side effect, which for a public banner is exactly right. Your matching avatar is the sibling page: 400×400.