One of the things I wish I had done differently as I started Bible journaling is to have been less afraid of drawing pictures in my Bible. Because I have never thought of myself as artistic, I almost completely avoided drawing anything beyond the most basic shapes in my Bible. While this allowed me to focus on my hand lettering, I was missing out on adding those visual elements to my pages that have now become a big part of my "style."
So...go for it. Give it a shot. Find a sketch or doodle that you like and pay attention to the details. What makes a picture look 3D? What sort of coloring and shading gives you the effect that you like? How do scale and perspective affect what you're looking at?
Happy journaling, friends.