Tran, if the pictures are stored on your computer hard drive, you can up load directly to FTT with out using the other site. First, re-size your picture so the pixels of the longest side do not exceed 750. Then click on the "tree in the picture frame icon" located the top of the reply window. Select "From Computer". Then "Select Files". This will allow you to browse the files on your computer. Find the folder in which the pictures are stored, then select the picture you want. Click OK. Then click "Upload". One post can contain as many as ten pictures.
If you are using a smart phone, there will probably be a different method. I don't have a smart phone so I don't know how, but someone else can tell you. Just ask.