Kindle Fire treats masthead logos differently than its e-ink cousins, and they end up not looking as good as on e-ink readers. The Fire automatically scales the logos and color-inverts them (so black becomes white, red become turquoise, etc.). The logo is displayed an an almost-black background (it's actually a slight gradiant).
The Fire also displays the publication front page on the Newsstand bookshelf, so this encouraged me to go looking for a source of these front page images instead of looking at the default calibre image.
The following code fragments can be inserted into your custom recipe to invoke a custom masthead logo and a front page image (if it's available).
Note that when you develop a masthead logo, plan for it to be color-inverted (so if you want the original color, provide the color-inverted version as the logo). The background should be R/G/B 211/211/211 and (after being inverted) it will blend with the Fire background to appear transparent. If you are really picky you can make the background pretty well perfect by using a linear gradiant (top to bottom) of 211/211/211 to 214/214/214. The size of the logo isn't all that important since the Fire will scale it, but logos at least 250 pixels wide will look better than smaller ones since upscaling doesn't work as well as reduction.
I have atached 4 Fire-friendly logos in a ZIP file (NY Times, Wall Street Journal, Globe and Mail, National Post).