By searching the net and this forum I believe there is no way to use URL scheme to launch the native photo app?
In case I am wrong let me know (it might change someday iOS6?)
Anyone knows another photo app that I can launch and be able to specify the path to a folder or picture
There is no url scheme available for launching the Native photo app of apple till now (10-November-2012).
You will get a list of third party photo app's that can be opened using the url schemes here
Make sure you are installed the third party application before launching it with the url scheme.