Making white area of a picture transparent.

[tm2383]

Hi, 
I practice ididology and would like to be able to superimpose the chart below over the iris of the eye on a separate layer. For this to work, I need to remove the white and make this section of the iridology chart see through, but leave the black lines intact. How do I do this in Gimp? Also, is there anyway to change the black to another colour, such as yellow, to make the black lines more visible when superimposed on an iris image. Thanks in advance !

[rich2005]

First be aware that is copyright, usually for something of that date, if the author is alive, copyright lasts 70 years.

However, You might be better cutting each chart out individually.

Make a selection, screenshot http://i.imgur.com/HPetQa6.jpg
Then Edit menu -> copy, then Edit menu -> paste as new image

edit: To make the black lines heavier, use Filters -> Generic -> Erode before Color to Alpha

On the new image use Colors -> Color To Alpha http://i.imgur.com/s0Jw4oa.jpg

edit: To change the color, put a colour layer on top, set the layer mode to screen example http://i.imgur.com/A8WLUas.jpg
an alternative is enable the alpha lock - little icon top right of layers dialogue - then bucket fill.