Do you actually want to superimpose text over some of your jpeg? Try, in the compose window, '''Format|Page Colours and Background'''. This should allow you to set a "wallpaper" style image as the background. I get the feeling that if it's a "letterhead" then it should appear at the top. I wonder if it's really as simple as trimming the image to wide but short, so all your text can be added beneath it. Sending a big image might harm you, as people may have size limits in email, or may suspect the non-text part is malicious. Email has its own conventions. Letterheads are not ordinarily part of it. A wide image sets a definitive minimum width for your document and that may be at odds with some email clients or viewing on small devices such as tablets or phones. I don't consider that to "look professional". If you want to do this often, look at setting it up as a template. However I think that doing it via the Stationery add-on is more flexible.