On Fremantle there is MAFW to allow multiple ways to "play"/"render" multiple sources. MAFW provides ways to combine both sources and renderers together and maintain play lists for them. Nokia's mediaplayer on Maemo5 uses MAFW internally.

If you have need to implement transitions, this could be one approach: write a renderer for MAFW that takes a playlist of photos, then displays transitions while playing out that list. You can have multiple renderers stacked as well.