Elmers and 3M both make a spray adhesive that is readily available and work well. You can clean graphics that have come loose and gotten dirty with soap and a scraper (I use a utility knife blade). I follow up with glass cleaner to get rid of soap residue, let dry well then spray the adhesive on the graphic (follow the instructions on the can for time to dry before sticking). Almost forgot - be very careful not to stretch your graphic when removing or cleaning. This works very well - my front fender graphic is on it's 3rd fender and is still sticking great.
if its just sticking up a little bit, superglue worked well for me. just cleaned the plastic and the sticker till it wasnt covered in dirt, then applied the superglue and stuck it down, used a rag to press the graphic down and hold for a minute then leave it.
ps. dont get it on your fingers, its quite a mission to get off again, dont ask me how i know that...