Grade 5 music theory questionWatch
But for this second picture ( minor 3rd) D sharp is in the key of B major which is also a semitone down
So I’m not sure how I am supposed to tell if it is a diminished or minor interval since they both go down a semitone from the major?
In the second image you have a 10th interval, aka a compound 3rd (as it is an octave + a 3rd). As 3rds are not one of the "perfect" intervals, you would need to go down 2 semitones, not one, to reach the diminished version of it. Therefore, as this only goes down 1 semitone, it is just a minor 10th (or compound minor 3rd).
I hope that helps