Often when I add pictures, drawings, etc. to my art journal, I just slap them in any old way. I work the text around them or over them (an opaque white pen is handy for this) and I don’t worry about whether the page as a whole or the two-page spread looks coherent.
Sometimes, though, I like to do a little more planning. A grid layout is a simple plan for where text and images go on your journal page. Like this: