if Flickr and Instagram don't seem like the right fit for hosting images for your Timeline, you can use the free image hosting service imgur.com
Uploading images is easy, but at this time, TimelineJS doesn't know how to turn the URL for the page which shows your image into a URL that can be used to include only that image on a slide. Uploading images on Imgur is pretty straightforward. Once you've done that, copy the "Direct Link" and use that in the "media" column of your TimelineJS spreadsheet.
Since this was originally written, Imgur's terms of service have changed. To use Imgur to host images for TimelineJS, upload the image to Imgur, then copy the URL for the Imgur page from your browser's address window. Note that this will result in your image being presented in an Imgur branded frame; while the method described above in strikethrough text may appear to work and avoid the frame, that is only temporary. For your readers, and for you after your cache expires, the images will not load, because Imgur will return a 403 "forbidden" error when the browser tries to get them.