With this version there are a set of breaking changes that must be considered when upgrading from a previous version.
Default image
Ind the previous version, if no valid quality was set or available, the fallback was the source of the image. In this version the fallback is configuable DFO 4.0.0 - 3 Setting up Optimizely
Look for the property MediaElements. The default mediaFormatId is 2. If you want the source, then you can append /source to the url. like this globalassets/digizuite/997345-en-butterfly-1-1535829.jpg/source.