you dont need an alpha channel to do this, when a format like tiff is connected to the colour of a matterial if an alpha is detected it will automatically hook it up to the transparency for you. You can go into the connection editor and delete this automatic association at any point.

If you bring in your texture jpg to the hypershade you will see a whole group of colour math nodes which will give you the result you want without working with photoshop. To make darker areas more transparent you need to invert the image so use an invert node then if you want to have more control play with gamma/ contrast nodes to get the result you want. A nice one to use is the set range node. when you hook this up you need to set the original range coming in and then the new range.

E.g. the orignal image will have a range of 0-1 so if you set the new range to be 0.5 to 1 the image will be alot brighter. Note you can scale this out of visible bounds to doing a range of -1 to 1 will make everything that was below 50% gray black. This is usefull for getting nice control.

Just hook the output of this into your transparency, use the ipr to tweak your values.
You can use alpha is luminance but you might find it easier for the brain to use rgb to luminance node.

Hope this helps.