Crop an X header image
The X header is a strip three times as wide as it is tall: 1500 x 500 pixels. This page opens on that size.
SnapCut crops and sizes the photo only. It does not check your face, expression or background.
Your photo never leaves this device. To check, open your browser's developer tools, choose Network, and crop a photo: no request carries it.
X lists the header size, 1500 x 500 pixels, in its help article on customizing a profile, and SnapCut links to it with the date it was checked. The ratio is 3:1, so only a thin horizontal band of a normal photo fits inside the frame.
To get a good result, start from a wide picture with the subject along the middle line. Zoom out all the way to see the widest band, then use the position sliders to slide it up or down. Rotate left and Rotate right fix photos saved on their side.
Your profile picture uses a different shape. The picker also lists the X profile picture at 400 x 400 pixels; open the Profile pictures view to see it next to the YouTube, LinkedIn and Facebook ones.
The export is a JPEG or a PNG at exactly 1500 x 500 pixels. A JPEG keeps the file small. A PNG keeps sharp edges on graphics and text, which can matter for a logo or a screenshot. Switching format never changes the pixel size.