One of the most intriguing questions about book publication is, who gets to choose the cover?  Like it or not, the publisher does.  You are, after all, the writer--not the designer, photographer, artist, or whatever.  They hire people who are these things to do covers.  This is especially true if you are a new writer.  In time and by mutual arrangement, you may be given cover consultation, though it may actually have to be in your contract.

• ADVICE. Relax about this, if you can.  Most publishers will listen to your ideas and your concerns, and do a creditable job of paying attention.  You may not get what you want, but there are after all reasons a publisher is a publisher, not the least of which is marketing your book.  This is their livelihood.  You can help, and you should help, but you should also be writing your next book.
  
• It's in everybody's best interest, however, for everybody to be happy, and everybody includes you.  Publishers know this.  They want to look at your next book, after all.  So, if you've got a cover idea, put it forward--even if they don't ask.  It's okay.  But the call is theirs.