Mail encodes attachments using a system called "AppleDouble." That means, when you send a file using Mail, it breaks the attachment into TWO files (one that contains the actual picture and one that contains the special Macintosh information like the icon) and sends both files as two diffent attachments.
This confuses some mail readers, like AOL.
You can still receive the picture using AOL. You have to download the file attachment, and then run a program that can decode the attachment. On the Mac, the programs "Decoder" and "YA-Decoder" can decode it and give you back the picture. On the PC, many programs exist to get the file nback; look for "MIME decoder" on shareware.com or download.com.