Vanishing Pictures

I’m having a problem with my entries – I put picture files into them by cutting and pasting the line of code it gives me when I upload them, then the blog shows them for a while, and then they vanish, though the line of code remains in my post.
Is there some way that I can make pictures stay? I’ve adjusted the allowable size, checked that they’re all .jpg, and still no luck in making the pictures come back. The line of code is still there, but no pictures on the blog page.
Any thoughts?