The attachments have size limitations in both dimensions and file sizes.
You can manage them with the popup box and select the files you want to upload by browsing them on your PC.
The files dimensions in pixels have a maximum limitation of 780x780
If your files are lager than this you will have to reduce them with software like mspaint on your side before you upload them.
One trick is to highlight the pictures you want to post, mail them to yourself and save the attachments. You can set Windows to shrink the pictures automatically when you do.
If the picture you want to post is online, you can use the picture icon in the composition window. I have attached a picture of the icon....too bad I couldn't insert it
