You can help Blender to find images by specifying folders for searching them. To do it, select item "User Preferences" from the main menu (see the picture below).
The following window will appear. Press button "Add a path with subpathes".
The file opening dialog will be shown. Find the root texture folder and press the "Select path" button. Only a directory path is used. A file name ("untitled.blend" in the picture below) is not used.
If that's all right, now you can see a list of all folders which will be used to find images when you'll import any .3ds or .asc file.
If you want, you can edit this list with other buttons.