I checked pack all into .blend to send a friend the file. But when he opened it he couldn't load background image. How to pack everything into blend file so that will be possible?
Unfortunately, there doesn't seem to be an easy way to see which image files are packed into a blend file, except to individually load every file into the UV editor, select the Image menu, and see if it offers the option to pack the file. If it doesn't then it's already packed. If it does then you should select that option, and when you've checked every file, save the blend file to finalize the procedure. You can then reload the same blend file and they should all be packed.