Ok... I was running out of ideas so i've "borrowed" this theme ( and even the text) from here. Any doubts can be clariified visiting the website wich contains some examples.

Elements of Design
"We are looking for photos that showcase line, shape, pattern, form, texture, perspective, etc. They can consist of any subject matter.
Perspective
Line
Pattern
We are also looking for images that demonstrate cool photographic techniques, such as bokeh and motion blur.
This category is NOT for:
Graphic design illustrations made in a program like Adobe Illustrator and extreme digital creations do not belong in this category. Here we are looking for the elements of design - line, shape, pattern, etc."