OK, if they are pictures on your computer or external drive connected to your computer such as a thumb drive it's really simple.
Click on the paper clip icon above your new post - it's next to the smiley face.
Then click on "Browse". Once you've found your file either double click on it or single click and then click "Open".
Next you need to click the "Upload" button. It is important to wait for that to finish and not close the window until it does. You're picture will appear as an icon below a "Current Attachments" heading that will appear.
You can also upload a URL.
Good luck!