Have you got them hosted elsewhere on the internet (like flickr or facebook?)
For most sites you can right click and select the code (copy image URL), then come back to this site and select the image button and paste the code into it.
If it's on Flickr you need to make sure you select the right option. Go to the image - above it you can see an envelope, facebook logo and a twitter logo - select the little black arrow for more options - select 'grab the html/BBCode' option and then copy and paste the code into the image box, as before.