how do I create a template letter with a jpeg logo picture as letterhead?

Depending on the version of Pages, each have letterhead template examples, or you can roll your own.

Start a new document with a blank word processing template. For each added image and Text box, disable object wrap — so you can overlap them. Set the font attributes in the Text box(es) to your needs, scale the image, and position with the Text box(es) the way you like, and then group all. Save once into your Documents folder as a backup .pages document, and then choose File menu ▸ Save as Template… .

Modify an existing similar template, you can reposition/resize the logo and text and tweak it all to your liking before you do as Viking suggests, Save As Template.

JPEGS btw are not really suitable for logos which usually are graphical with sharply defined lines. A PDF or Tiff would be better. JPEGS are inherently blurry.

Peter