Try sending a smaller-sized and low-resolution photo, and see if that is transferred.
You can attach a photo within the iPhone mail app, too.
I’d guess you can attach photos, but you just can’t attach the particular photos you want, or can’t attach as many photos as you want. Probably because AOL limits the sizes of attachments. (I don’t immediately know what that attachment size limit might be fore AOL, but pretty much every mail provider has some size limit for messages.)
Apple offers a service known as Mail Drop for larger attachments: Mail Drop limits - Apple Support. Mail Drop temporarily stores the message attachment in iCloud+ storage, which bypasses the mail provider storage and storage limits.
I’d expect the Windows default Mail client can connect to AOL mail. How depends on which version of Windows and which mail client.
The selected default mail account is unrelated to whether attachments will or will not send, though it can be a problem if another app selects a mail account with restrictions on transfers.