Unfortunately, the photos taken by the officers of the mark on your wife’s chest can be used as evidence against you.  Oftentimes in domestic violence cases the alleged victim refuses to testify or is unwilling to cooperate with the case.  However, that in itself does not preclude a prosecutor from pursuing criminal charges against you.If a prosecutor feels that the cases warrants it, then he or she can still take you prosecute you despite the alleged victim’s wishes.  Your wife should know though that she has a statutorily-protected right in California to not testify against you in criminal court, a right which is sometimes known as the marital or spousal privilege.
These situations are not uncommon, but if you are charged with domestic violence there are things you can do to protect yourself.  The first is to hire an attorney who can assess the case against you and work to get it dismissed or reduced, if necessary.  Second, if the argument was a one-time occurrence, and you and your wife are currently taking measurable steps to work things out, then you can have your wife explain that to the prosecutor.  Depending a variety of factors, a prosecutor may be willing to drop the case if he feels assured that the incident was not that severe and that the likelihood of it happening again is slim to none.  At any rate, you should talk to an attorney.