First of all you need to make sure that you have saved the pictures as Jpegs, gifs or Pngs.
Then there are two options.
You can either click on the advanced post reply option and then scroll down to the bottom half of the window where there will be a button with "Manage attachments" on it. Click on it and a pop up will appear where you can select the picture you want to upload. Then click on upload and eventually it will finish, then you can close the popup and the picture will have been attached to your post when you post it. This way is best if you are only posting one or two pictures.
The alternative is to use the uploads link at the bottom of every page which also opens a separate window where you can select the file you want to upload although this time once its uploaded you will have to link the image to your post by using [IMG ] [/IMG ] tags without the space around the link, something like:
[IMG ]http://www.civfanatics.net/uploads8/yourfile.JPG[/IMG ]
This is the best method if you want to attach more images as each picture can be linked separately into the same post and positioned correctly.