This can be a low polygon version of your orginal model. Meshlab's quadric edge collapse decimation is my prefered method.
The default options should be sufficient so click on ok.
You should now see your new UVs layout in the UV/Image Editor window.
It should automatically be placed at the same location as your first obj file overlaping it perfectly.
You should now see the texture placed correctly for your UV unwraped obj in the UV/Image Editor window.
I use Maya because unfortunately blender's texture paint will tend to overspray the paint to other UVs and can cause you to paint in two different locations at once. Maya does not have this problem. You can now paint over the magenta triangles!