OK....calm down...
'Twas me who sent the email.
A wallpaper needs ideally to be a zipped bmp for the 'file', and a full-screen size jpg for the 'preview image'.
What you uploaded was a small pic....heaven knows exactly what size, but you ain't the first nor the last to 'get it wrong'....
Just save your pretty picture as a jpg, as well as the format you drew/created it in....and upload the jpg version as the 'preview', zip the bmp, jpg, or multiple resolutions into one zip and upload as the 'file'.
That'll do it...