There are many beautiful images of microalgae on the internet. Some of them have a CC attribution. Can I use these images in my publications and how do I correctly cite them?
There are different CC licenses, so you need to check which one applies to each picture. See here what is allowed for each type of license: http://creativecommons.org/licenses/
Often (but not always), you cannot use them in any kind of commercial product. Papers from journals which are financed by subscriptions, would fall into that category. However, in many Open Access journals, like PLoS One, which uses the CC-license themselves, it would likely be ok to use the images.
Correct citation would include the author's name, source (with URL if possible), and exact mention of the chosen license. Of course you need to consider whether the source is trustworthy enough for your purpose, for example if you can rely on their sample identification.