Hi Tumai, I was going to say Good Evening but its Good Morning where you are! Lol
When you post a message it gives you a button on the top right saying "upload pic" If you click on that it'll take you to browse your documents so you can just click on the one you want. Its good because it automatcally resizes your pic for you so it fits etc
If you want to put it in your gallery again its the same principle, but you need to resize the pic yourself this time. In the program that you keep your pics if you go to edit or tools it gives you a option to resize your pic. You need to make your pic 600 pixels by 400 pixels (I think that's the right size, it will tell you if not) and then when you are in browse select that pic and it will be put in your personal gallery.
If you still get stuck, give me a shout (lol) pm me and I'll try and help you out!