Photos - if they are allowed at all - are limited to 128 kbyte. You need to shrink the size or resolution. Steve has a Picture Resizer on his main page that works well. Microsoft Office also has a way to reduce picture size, and other online resources can do it.
If you want a better picture, you need to load it on another site like Google+ or PhotoBucket then paste a link to the site using the Mona Lisa button above (bottom left button - titled "Insert Image" if you hover over it). Then there is no limit since it doesn't load onto Steve's server.