A photo can be added to a post before or after adding text to it, the photo can even be posted without text in the post. Before posting a photo you should first resize it to no larger than 800x600, if you are not sure how to resize the photo, please read How to resize a photo.
Now that you have added text to the post (item-1) and ready to add a photo select the Browse button (item-2), see below.

Once the Choose File menu appears browse for the photo to post, once selected (item-1) select the Open button (item-2) which will close the menu, next select the UPLOAD button(item-3), (see below). At this point the photo has been added to the post and will appear as a thumbnail at the bottom of the post once posted.

If you would like to specify where the thumbnail is located in the post select Manage Current Attachments, make sure the cursor is where the thumbnail is needed and select the Insert attachment into text editor icon (see below).
NOTE: The second icon with the red X is the Delete attachment feature, select this icon next to the photo you have loaded to delete it.

Selecting the Insert attachment into text editor icon adds a text string indicating the location where the thumbnail will be place in the final post (see below).

When finished inserting the thumbnail of the photo select Post New Topic or Add Reply depending on whether a new Topic is being started or a Post to an existing topic is being added (see below).