Should I alert my colleague not to let anyone see my old photos that I took with a camera during the period when I was not wearing the شرعي (Shar'i) attire, or is this thought a form of وسوسة (waswasah - intrusive whisperings/scrupulousness)?

If you can communicate with your colleague who took your picture, ask her to delete the picture or prevent men from seeing it. If you cannot communicate with her, do not worry about this matter. Stay away from whisperings (waswas) and seek Allah's help.