Enter the comment you want to make...
Then click the attachment icon. (It looks like a paperclip)
A small window will pop up.
Click on "choose file".
Once you click on "choose file" another window will open up for you to navigate to where the image is located on your computer.
When you have located the image, clicking on it will automatically enter the file name of the image. Once that happens, click on "open".
You will then need to click the upload button. If you do not see it you may need to scroll to the right of the small window until you do.
Once that is done...
Simply click on the paperclip icon again and select the image you want to insert by clicking on the file name. This will insert the image to your post.
For help inserting images using the smart phone viewer, please see the following thread.