If I recall correctly, you can't just forward it if you're using the stock messaging app (if you can you should try long pressing on the picture, but I don't think that works). If you're using the stock one, you'd need to save the picture and then send it to someone else.
If you're using Hangouts for SMS/MMS, however, you can just long press on the picture message and select FORWARD from the menu that pops up.