How do I create a rule to prevent a user from sending a message via the Privatemsg module? As a courtesy to registered users on my website, I require all users who send private messages to have a profile picture. 

I already have system messages throughout the site to encourage them to upload such a picture (for example, when they log in, or visit a user's profile) with a link to their account edit page where they can upload such a picture.

But how do I block them (without banning them of course) from sending a private message if they do not have a picture (in other words BEFORE the message is sent?) 

Fall back scenario: If I cannot stop the action before it happens, how do I prevent the message from being delivered, with the appropriate warning to the sender of course?

The message is sent by the Privatemsg module, not the Rules module.

Any idea?



UPDATE

I have tried both solutions proposed below (jeroen and Pierre Vriens) and both did the job.

Pierre's solution below works well and is easiest to implement.

For those who need guidance in implementing it, here are the steps:

1 - Create a new role. I called mine "can send private messages"

2 - Give this new role the "send privatemsg permission" only. Remove the same permission from the authenticated user permission (and other roles that have it and shouldn't.) At this stage, only those users who have the new role will be able to send private messages. The others will be sent to a "you do not have permission to access..." page.

3 - Create the first rule: Give it a name and select, under User, "an account is created." Save it. Create a second event and, still under User, select when an "account is updated." You now have two events in the same rule.

4 - Create the condition: Select "Data value is empty." Then drill down and select the name of the field that should contain the main picture (in my case, it is "site:current-user:field-user-image".) Below that, check the Negate box. This means the rule will check if the field is NOT empty.

5 - Create the action: You want the role to be granted if the field is not empty. So select "Add user role" under User. Data selector is "account" and select the value of the role (it's the name you gave to the new role.)

And that's it. To make the user lose the role if he removes the picture, you need to create a new rule, albeit a little less complicated. I called mine "disable private messages send." It only has one event: "After updating an existing user account" The condition is, again, "Data value is empty" but DO NOT check the Negate box. For the action, it's pretty straightforward. Like for the first rule but select the remove role instead.

For this rule to work the user must go into his account and click the save button. This is ok with new accounts but it may be a drawback if your site already has many users. But eventually, they will all update their account one way or another and the rules will be implemented then. If only a few users, the administrator (user #1) can go into their accounts and click on the new role box to grant the role.

See my second update below.

Again, thank you to Pierre for his insightful approach.
I don't think you can use the Rules module directly for this, because the only action that is provided by Privatemsg is called when a message has been sent. Therefore, I suggest you write an easy piece of code.

When a user hasn't set a profile picture yet, the field is empty or displays the default picture if you picked one in your configuration. In both cases, $user-&gt;picture has value 0. You could use this fact by hiding the sending form for a private message with the following code in a custom module:

function MYMODULE_form_privatemsg_new_alter(&amp;$form, &amp;$form_state, $form_id) {
  global $user;
  if ($user-&gt;picture==0) {
    hide($form);
  }
}


As suggested in the comment below, you can also use $form['#access'] = FALSE; instead of hide($form);. You can add a line of code that, for example, displays a message Please add a profile picture! or whatever you want in the if statement.