advice about putting translucent images on gimp

ok so i saw yalls bad arse jobs with

and this

so i wanted to get a pic of sanford like those stadiums and put a picture of Uga VIII in the background up top like Nova in the background of the AU one. problem is, idk how to make images "translucent" or w/e. idk how to make them faded

also i want to add an awesome texture like in those so its not just a blurred or on a grey scale. i cant find it but there was a sick picture of patrick petersen where it looked like it was raining and he had his hands up

any advice boys?