I uploaded an image that I've used on the discussion forum here before to Flickr using a CC BY SA 2.0 license.
This is a decorative image, so according to the BC Open Accessibility Toolkit I actually would not need to add alt-text in this case. By leaving the alt-text field blank, this signals to the user that there isn't any contextual content worth noting and to move on.
However, if I were to include alt-text it would go something like this...
A wooden boardwalk in a marsh surround by silver birch trees. It is daytime and the skies are the same colour as the surrounding birch trees -- grey. Wow, that was a lot harder than I thought it would be, and I'm sure it could be better.
I tried to keep my description short and to the point
I tried to leave out redundant information that would likely be included in the text (i.e. the location of the marsh/Sackville Waterfowl Park or the date that the photo was taken)
I also tried to leave out unnecessary information (i.e. 'this is a photo of', 'here we have a photo of')
Today's lesson was really informative, thank you. I'm ashamed to say that I'm usually pretty bad at remembering to include alt-text for images in my presentations, on my blog, while sharing graphs in reports etc., etc. So will try to be more inclusive while creating materials in the future- from the get go.