Fianchetto,
I will be glad to help you post pictures, if I can. I have posted quite a few here. I usually start by reducing the size of the picture to about 800 or 1000 pixels wide. It's mostly width that matters for viewing. I use photoshop, but you can also use gimp 2, which is free software. You probably don't really need to do this step, as the board software will maybe shrink it down anyway. In any case, the picture is saved on your computer to start.
The software for the board has changed a little bit, so it can be a little confusing to try and follow the instructions that are in David's link.
Click Gallery.
Click Members Gallery.
Click Upload.
Select Create New Album.
Name it something you like.
You can probably follow instructions to load the picture from there.
Once you have some pictures in your album, to include it in a post, click on the desired picture.
Click Options.
Select Share Links from dropdown menu.
Highlight the BBC code and copy and paste it into your post.
That's it. Hope it helps, let me know if you have any questions.