Commercial use is defined as use “primarily intended to obtain a commercial advantage or monetary compensation”. However, that compensation can be direct or indirect; therefore, if you use an image in a blog post or on a website affiliated with a for-profit company, the use is commercial. Commercial activities would be: production of merchandise, images used in movies and television, in publications that are sold and commercial advertisements and promotions online or offline. The use of images on or in anything created with the intention of making a profit would be considered commercial use.
Did you know that most images, music and other media on the Internet cannot be freely reused for your own purposes? In most cases, unless you have permission to use this type of content, doing so is illegal. CC BY allows people to distribute and mix content, even for commercial purposes, as long as they credit the original author. If you use the content for general research, even if not for a specific purpose, and you or your organization generates revenue, that's considered commercial use. A European Union directive passed in 2001 finally became law in Great Britain in October 2003, ending the “fair use” clause that allowed “small” parts of a work to be copied for commercial purposes.
Basically, if there's a shred of commercial interest in what you're doing, then it's considered commercial use. It allows people to download and share content whenever they provide credit, but it prevents them from changing the work or using it in a commercial way. Licensees can always contact the licensers to request permission to use the work for commercial purposes. Non-commercial means that something is not primarily intended to obtain a commercial advantage or monetary compensation from a person or organization or to gain a commercial advantage.
The commercial use of an image would be to reproduce it in any way that is primarily intended or directed to obtain a commercial advantage or monetary compensation. CC BY-SA allows people to republish their content, including derivatives, even for commercial purposes. You are always free to locate and contact the rights holder to request permission to use an image for commercial purposes.