Jay,
Welcome to the Gauge. Go to the top of this forum and read the tutorial "adding images". That outlines the file type and size, and pixel requirements. Your best bet is to use a photo editing program to get the size and density you need. Then just go hit the "browse" botton at the end of the compose window, find your picture on you HD and the post your message and picture. If you did not meet the criteria, you will get a message telling you to reduce the picture size, or pixel count.
Oh, "Preview" will not show your picture.
Don