.jaskin is just a .zip file with a different name. Just change it to .zip and expand it, then rename the folder and finally add that folder to the user’s skin directory.
However, there is an alternative way, but it would need doing with each new install of jAlbum.
From
the Skin and Style section of the Welcome page
‘If you want to revert to an earlier version of a bundled skin then you will need to remove the bundled skin before you can select the older version. To delete the bundled skin select it and then click on the skin info button, confirm it is the bundled version by checking its location and then click the delete button, see 'Skin info' below for details.’