Can someone simplify my texturing problem?

So, I'm using blender for the modeling and texture mapping. I've been able to figure all of that out, including exporting a texture map of the unit for me to "color in". The program I have right now for any art related work is paint.net. What I was hoping to do (and forgive me, I'm new to texturing and art related work, so I'm not sure what terminology is familiar to everyone) was use paint.net's layer's feature to have one layer be my exported texture map and another one under it to actually be my texture... so I can essentially "color" between the lines without affecting the lines themselves. The problem is that blender exported the texture map as black lines on a white background. In order to take advantage of using layers, I need to find a way of erasing the white (make it transparent).
So my question? Does anyone know how I can easily erase the white background while leaving the black lines untouched? Currently I'm doing it the old fashion way of zooming in and going pixel by pixel when close to the black lines. Is there a program out there that allows me to specify a certain color to erase while leaving all the rest alone? Or anyone know how to do it with paint.net?|||Haven't used paint.net myself, but I'm assuming it has the sense to include all basic features like the major labels have. First question is, in what format did you export it from blender? Does it include an alpha channel? See where you can load any alpha channel as a mask for that template (texture map lines) if it has one.
If there is no alpha, and neither can you get one from blender (you should be able to, try exporting in .tga for example), then there are a couple of trick to try:
1) copy the image in paint and paste into your new layer using the paste-with-transparency (should be featured in paint in some way) where the currently selected secondary color is interpreted as transparent. You may need to add a small corner crosshair to properly place the now transparented map.
2) just make the new layer as you did and use the magic wand for example to select white areas and remove them.
Using alpha channel would be the right way to go, and if that is not possible then the paste-with-transparency...
Hope this helps even some.|||I have the same issue with Blender, even though I export the UVs in .tga I don't see any option to have transparency, so it always comes out black lines on white background.
I figured out how to erase the white in Photoshop, Just choose the menu option Select > Colour Range... and it gives you a colour picker to choose the white. Then you can just delete it. Maybe there's a similar thing in paint.net?
what Kirvesnaama said could be what you need, or even, after a quick google I got this: http://jasonmray.com/code/removeWhite.php, it appears to be a white removal plugin for paint.net..?|||Thank you both for your help. The remove white plugin seems to accomplish what I need for this purpose.
Kirvs, I'll look into your suggestions as I can later. Thank you for your input.