Villi,
There is a limit on the size of photos you can upload, so first you need to make sure your photos are not too big. I usually re-size my photos to 72dpi and size them to roughly 600x400.
When you have your photos ready, click on the POST REPLY button type what ever message you want to write to describe your pictures. Then scroll down the page and you will see a button named MANAGE ATTACHMENTS. Click on that button. A small window should pop up on your screen with 3 empty lines and 3 buttons that say BROWSE. Click on one of the BROWSE buttons and it will let you search your computer for your photo. When you find it, click OK or OPEN. You can post up to 3 photos in each post. After you load your photos with the BROWSE buttons, you need to click the UPLOAD button to copy the photos to your post. After that is done, close the window. Then click the SUBMIT REPLY button.
The instructions I gave are what I see using Firefox. If you use a different browers, your instructions may appear a little differntly but probably will work the same.