Banners are the hardest images to get right, because they are displayed at completely different shapes depending on the device.
Crop to the banner's intended proportions while keeping the subject and wording inside its usable area. Check small text after resizing and any additional crop at the destination. This tool changes dimensions, not the banner design.
Locked resizing keeps each image's proportions. Fixed dimensions can stretch images; crop to the required ratio first.
The encoder creates a new file. Upscaling cannot recover detail that the original did not contain.
A banner is far wider than it is tall, perhaps four times as wide. Forcing a normal photo to that shape in one step stretches it. Instead, crop the photo to the banner shape with Crop & rotate, using Free to draw a wide, short box, and then resize it to the exact size.
Keep what matters in the middle of the crop, because some sites trim the left and right on small screens. Check the result at the size your page will show it before you upload it.
No. Check dimensions here, then use Compress to a size if the destination separately limits file size.
JPG, PNG and WebP always open. Other types depend on your browser, so convert them first with Convert format. A very large photo can run out of memory on a phone, so try a computer.
You can force the exact width and height, but that stretches a photo of a different shape. Cropping to the shape first and then resizing gives a natural result.