Don't stress about it. I also just took a selfie of myself and took a head shot of my husband with my phone for our recent cruises. I did not change size or anything. I just uploaded the pics into the DCL app. Pic should be from shoulders up, eyes visible, nothing on head, against a blank background, nothing else in pic besides you.
I think the app and website can be finicky when uploading. I have had pics upload really quickly and really slow. Just depends (I think) on how said app or website is feeling that day LOL. For instance. yesterday I helped a friend do her online check-in. She sails on Saturday so technically not many people should be checking in this late. She has two rooms with 3 people each in them. Some pics uploaded right away, some just spun. Once or twice we had to exit out of app and go back in. Just save everything along the way when it says "save".
MJ