Change tab bar icons and image placeholder in Workshops/Presentations

[imilicevic]

• ikone su ovdje: https://drive.google.com/drive/folders/1wokpMMHV_yUZaQaYOfbVdA3ogpVEVFBB
• placeholder image za Lecturera u Eventima: https://drive.google.com/drive/folders/18xjdDPQaSdm_-SMB7qrmLxXpVdZvHHJP