Teams custom backgrounds are one of the better-specified image uploads on any platform: 1920x1080, 16:9. When an image does not match, Teams letterboxes or crops it, and when it is too heavy the upload is refused with a message that does not explain which rule you broke. Most people arrive with a photo straight off a camera, which fails both rules at once. So the work is mechanical, and doing it locally is faster than round-tripping through a web uploader. Skwish resizes to the 1080p frame and compresses to a sane file size, showing the result before you save it. Prepare a folder of options in a single batch, check each preview, and keep the ones that look right — nothing leaves your machine.