Image includes photography, drawing, paintings, clipart, etc. There are exceptions to all these rules. When in doubt err on the side of caution. These rules may seem unnecessarily cautious. It is my contention that it is better to universally apply best practices rather than attempting to use them only in instances “when they really matter.”
Nothing on the internet is free.
Every image has a copyright, and it is owned by someone. Regardless of your intent, don’t use it unless you obtain a license. If you cannot locate the rights holder, don’t use it. If you think it is in the public domain, don’t use. The rule of thumb is, don’t use it unless you explicitly have the appropriate license. Even if it is a license you believe to be “okay” (Creative Commons for example) read the agreement, the language and rights vary. Always assume image owners are litigious. Using images without permission may cost you your client, your job, your money or possibly all three.
When using free or Creative Commons type licenses you are still legally responsible. For example, if a model release or property release was not obtained and a dispute arises you may be responsible. A license does always ensure legal protection. Be vigilant.
Whenever possible make it yourself.
Not your friend or colleague, you. (Unless you can get the transfer of copyright) Shoot it or draw it yourself. It is usually worth the effort. If it the project doesn’t work out you can always use the image again. Short of spending money on exclusive rights, this is the only way to ensure that the image is only being used on your project. It also sidesteps any unforeseen costs when, for example, a project expands and you need additional rights for a larger scope or quantity.
You don’t buy images, you buy the rights to a specific use.
Image creators and stock websites make money by selling the same image over and over, dozens, hundreds, perhaps thousands of times. Be aware that others may be using your favorite image*. Many people using the same image means they all have limited rights. The trick for you is to ensure your license matches your use. It is not always intuitive. For example, you may purchase rights to line art that is obviously intended to be used as a logo and the creator may explicitly state it is intended it for logo use. The kicker is that you are most likely not buying the rights to use it as a logo. Despite common sense, the license says single use only. A sign? Yes, Business cards? Yes, A sign and business cards? Sorry no. It sounds sketchy and misleading because it is. Confirm applications, quantities, and media. Don’t assume anything.
*If you need exclusive rights you need to pay for exclusive rights.
Fair use does NOT apply to you. (Most likely)
“Fair” in this instance relates to the public good. Fair use applies when you engage in activities like a review, a commentary or a parody. Transformative works can also be deemed fair use. Unless you are engaging in commentary or artistic activities, your time is better spent creating an original image rather than attempting to “transform” someone else’s copyright.
TLDR: Make it yourself, if you can’t, then use an appropriately licensed image from a reputable vendor. Even if it is “just for your blog”, get a license. (Licenses don’t always cost money) If you are making money off an image you owe someone money for the image. Read all the fine print.