Some Mac users have problems uploading pictures. Here are some Mac tips by painterbear:
iPad Picture uploading
Members uploading pictures from their iPad are finding that every picture uploaded on the same day overwrites the previous one. This is because all pictures on the iPad have the same filename.
The work around is to use the Manage Attachments facility below the reply box and to put the pictures in separate posts
Files not seen in the Uploader:
The default file type in the uploader window is .GIF so you may not see your jpg files. You need to select "All Files" in the filetypes window: