This is most likely caused due to a bad path on your images. You will need to verify with a text editor. First check path of .mtl file is correct in .obj file. Then confirm that image paths are correct in .mtl file. Look for something like...
If there is a long file path, you can try putting all the files in the same folder and editing the .obj and .mtl paths to just the file names. Also SweetHome3D really likes .png files as images. So if you are using any other image format try switching to .png files.
Hope this helps.