Is it ok if I use my own pictures?

Sure, use your own source material all you want. Stick to what you do best so your portfolio is as strong as it can be. For instance, don't do a lot of sculpture or ceramics if those areas aren't what you are all about. For a good interior design portfolio concentrate on showing good design in your compositions and attention to detail. Design is present in any work you present, so plan things carefully. Also, let your individual style show through, but don't go for the shock value too much. Garish colors and wild ideas will grab attention, but may be a bit much for an entrance portfolio. Also, have some fun with things--showing some whimsy and sense of humor is never out of place.