Elfie, at the bottom of the "Your message:" box there is a green button that says "Upload an image" When you click it, you can browse your photos and just click the one you want. It will upload to your post. It doesn't appear as a photo but as a line of text and symbols inside square brackets. If you want to see what your post will look like with the picture, click "Preview" and you will see the picture as a thumbnail. When you hit "Finished", then it posts and everyone will be able to click on your picture to see it at full size.
It's similar on the database, look for the line at the bottom under "Adding Actions" in blue font that says "Upload a photo". Click on it (any line in blue is a link, btw) and the same thing happens.
Dave has made this site incredibly easy to use, so don't be afraid to click around and try things. It's pretty hard to mess up. If you get somewhere you don't want to be, the back arrow at the top left of your browser will get you back to where you started.