I ejaculated onto the bathroom wall and i'm afraid some of my semen might have spilled into the showerhead. if a female uses that showerhead to wash her vagina and there's semen residue left inside, could she get pregnant?

No. What you described would not result in pregnancy. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV vaccine. Wish you good health!

Pregnancy is not possible in above circumstances. Hand hygiene rules still apply and should be followed consistently.