Uploading has been explained before but I understand why you are asking.This is for windows operating systems:
Find your image it can be something already on your computer or something from the net.
If it is from the net you will need to download the image to your computer before being able to upload it into Sblows. To do this place your mouse over the image. Right click. Select "save picture". Remember the name of the picture and the file you are storing it in.
When you are ready to post the picture. Click the upload link. Click browse. Find your image by name, highlight it and click open. All that is left to do is click upload. Once the system gives you the link record it. You may either write it down or click and copy so you can paste it in your post.
After you have your picture stored and are ready to look at it. There is a preview button on the lower left corner. Use this to make sure everything looks how you would like. If not make corrections then click either preview or post. This action maybe taken over and over. Once you click post you are committed to making a post.
I broke this down and did not mean to over simplify it. There are a lot of us that had trouble to start with.
Hope this helps and you enjoy your posting experiences 