Are you actually getting any pain or symptoms?

First of all, it's not a great picture for assessing your posture as having a front/back and side view is best...and you can't really see your back from side on in that photo. Your posture doesn't look "awful" to me from looking at that picture, and in the picture where you're "flexing," you're not really flexing much at all, so it's difficult to see what you mean. Standing up tall and ensuring your neck is in line with the rest of your body and you're not slouching/letting your shoulders round will improve your posture. Imagine there is a string going through your head and sit up as tall as possible / use a rolled up towel or lumbar support when sitting to ensure you maintain good posture.