I'm only answering this in case someone else has the same question.
Pictures don't get deleted, but there is a maximum height of 200px. There's also a max width but I forget what it is - it's wide enough that you shouldn't need to worry if your picture height is less than 200px.
In addition to these restrictions, the picture will of course have to hosted somewhere on the internet. We host avatars here, but not other pictures. It's not polite (or legal) to take pictures from other websites without permission, and the other website might have a method of blocking the picture from being shown if you try to post it here.
Finally, do not post offensive pictures. We have relaxed standards around here and almost nothing is ever deleted, but pictures or posts that are solely here to offend or annoy are not allowed.