Recently loaded images(up to 5 days?) should be cached so that if the user goes to that date again, the image is loaded from cache instead of from an API request.
One way to do this would be to have a production index.htm and a development index_dev.htm. The index_dev.htm would be identical to index.htm except index_dev.htm will have the mock service set up.
A build process (grunt/gulp?) can automate the creation of index_dev.htm.