Thanks for reaching out to us!
Currently there isn’t a “line tool”, however there are a number of arrow shapes on the Stickers tab that you can add to your photo. When you click one of these stickers after adding it, a Sticker Properties dialog appears and you can use the Eraser tool on this to erase the head off the arrow and just leave the line.
Let us know if you have any other questions!
Regards – The Ribbet Team