With the high resolution and editing capabilities of mobile phone cameras today, you can create great images with just a little practice. Try to use natural lighting, keep your shots simple with one primary point of focus, and use your available editing tools (crop, contrast, and hue) to make final adjustments. It's cost-effective, and you sidestep any issues of copyright by using your own original material.
If you plan to obtain your imagery from others, you'll need to consider cost, credit, and usage rights
. Look online for plentiful, low-cost stock image resources where you can quickly filter and find the right photo or illustration to complement your copy. These sites will list the requirements for purchasing rights or crediting the source so you can ensure you're staying on the right side of your associates operating agreement