There are a few options:
- Edit the MTL file by hand so the file paths to the images are correct. You can open it in a text editor.
- Move the image files into the same folder as the MTL file (I think that should work!)
- Go to the Library and Content preferences. In the User Content Folders section click the Add Folder button and choose the folder with your images. Click OK to close the Preferences dialog.
You will need to reload your object after doing this.
Regards,
Jo