is there a way, to change the image data in runtime
There should be a function to get a pointer to what was your cRGBA. You shouldnt recreate texture data at runtime though, better make some textures at startup to avoid huge slowdowns.
targas can save alpha chanells right into the file, right
Right you are, 32bit tga files have an alpha channel.
is glDrawPixels the best approach, or is there a better way?
glDrawPixels is SLOW, it's much better to use texture mapped quads.