One of the resources we like for custom image creation on a budget is canva.com (Links to an external site.). They have very helpful, short, interactive tutorials on their website under the “Design School” section.
Another resource along these lines that some people like is http://www.picmonkey.com/ (Links to an external site.) but I haven’t personally explored this one much.
Please let us know about the images you use and where you find them. Do you have another editing platform you use that works well?