Don't use the quick reply box. Instead hit the reply button. There is a button near the bottom where you can search your hard drive for the filename. Put a file comment in the next box and then hit add attachment.
If your picture is too big, you sometimes have to make it smaller. Here are the limits for pictures and the types of files you can upload.
Allowed Extension Groups and their Sizes
Images -> Maximum Upload Size: 100 KB
gif ief jpe jpeg jpg pbm pgm png pnm ppm
ras rgb tif tiff xbm xpm xwd