Okay, you need to know which file they are in. Or, for ease, pop them onto your desk top.
Then, when the box comes up, press select.
This should then bring up another box, with all your PC contents.
You then need to select the file/folder where the pics you want to upload are.
Select which you want to upload, then press upload. Wait until the picture comes up, then save. It will then be in your post.