You can make the background a solid colour (I usually make it hot pink or something), then load up Gifworks (there is a free online version) and select Edit->Transparify, then click on any part of the background and then any solid pink area will be transparent.
I don't know how to do this with any standalone program like Photoshop or Irfanview though, sorry..
You can also just leave the BG transparent and save it as a PNG out of Photoshop... that way you can have 256 levels of alpha instead of just one.
We here shall not rest until we have made a drawing-room of your shaft, and if you do not all finally go down to your doom in patent-leather shoes, then you shall not go at all.
We here shall not rest until we have made a drawing-room of your shaft, and if you do not all finally go down to your doom in patent-leather shoes, then you shall not go at all.