Kate, I'm almost as much a noob as you (I know HTML, but am a complete novice at ePub) and have also just finished my first "conversion" using Sigil.
Getting the cover right was a bit of a challenge, but I have a couple of suggestions:
1 - Use an image with an aspect ratio of 0.75 or greater. (That's width to height, for example, 600x800 pixels.) Doing that will prevent the bottom from getting cut off when the picture is expanded to fit the width of the screen. It'll be cut off if the aspect ratio of the screen is larger than that of the cover image, but you're not likely to run up against a device with a screen that wide in portrait mode. In landscape mode, all bets are off.
2 - Set the image width to 100%, which will force the reader to expand it to the full width of the screen. (In your img tag, include: width="100%".)
3 - Put a chapter break after the picture. I suppose that's obvious, but don't forget to do it.