How can I delete the "Recents" pictures when selecting a profile photo?
The process is a bit complicated.

To delete the Photos in recent, you need to first navigate over to:

~/Library/Containers/com.apple.ImageKit.RecentPictureService/Data/Library/Images/Recent Pictures


(Finder > Go > Go to Folder)

There you will find all your recent pictures. However, you can't just delete those pictures. You have to also delete it from the plist file available at:

~/Library/Containers/com.apple.ImageKit.RecentPictureService/Data/Library/Images/Recent Pictures/Recents.plist


Just open it in Xcode or any text editor, and delete the dictionaries under the array.

A picture in the plist will look something like this:

&lt;dict&gt;
    &lt;key&gt;cropInfo&lt;/key&gt;
    &lt;dict&gt;
        &lt;key&gt;cropViewInfo&lt;/key&gt;
        &lt;dict&gt;
            &lt;key&gt;PRS&lt;/key&gt;
            &lt;dict&gt;
                &lt;key&gt;px&lt;/key&gt;
                &lt;real&gt;0.0&lt;/real&gt;
                &lt;key&gt;py&lt;/key&gt;
                &lt;real&gt;0.0&lt;/real&gt;
                &lt;key&gt;r&lt;/key&gt;
                &lt;real&gt;0.0&lt;/real&gt;
                &lt;key&gt;sh&lt;/key&gt;
                &lt;real&gt;0.390625&lt;/real&gt;
                &lt;key&gt;sw&lt;/key&gt;
                &lt;real&gt;0.390625&lt;/real&gt;
            &lt;/dict&gt;
            &lt;key&gt;cropViewSize.height&lt;/key&gt;
            &lt;real&gt;200&lt;/real&gt;
            &lt;key&gt;cropViewSize.width&lt;/key&gt;
            &lt;real&gt;200&lt;/real&gt;
            &lt;key&gt;zoomValue&lt;/key&gt;
            &lt;real&gt;0.0&lt;/real&gt;
        &lt;/dict&gt;
    &lt;/dict&gt;
    &lt;key&gt;cropSize&lt;/key&gt;
    &lt;string&gt;{200, 200}&lt;/string&gt;
    &lt;key&gt;original&lt;/key&gt;
    &lt;string&gt;68937D05-204D-4AD8-A98F-2DB8E9EF945F.png&lt;/string&gt;
&lt;/dict&gt;


You can delete each dictionary/picture manually by looking at the string parameter.