Kathy - there are two ways to post a picture - you can upload it or you can use the "insert image" button and paste a link to the picture.
When you upload, it resizes it so it doesnt take too much server space. It also just puts a thumbnail which you have to click on to see larger.
To make it bigger, you can link to any size pic you want - you just want to be respectful of folks with slow bandwidth and dont link to crazy huge pics.
Hope that helps! 587