Valid, although I think @erjohnso's point in the description is labeling when the data on disk came from. I'll leave the decision up to him, I don't care either way. 

If I was to do it though, I've always preferred the syntax:

```ruby
 my_description = "Created with fog"
 if !source_image.nil?
  my_description = "#{my_description} from image: #{source_image}"
```