Welcome to the forum.
The best way to set a constant Duration for Imported Still Images, is to do so, BEFORE you do the Import. Go to Edit>Preferences>General>Still Duration, and set it there. It might be easiest to just Delete the Imported Stills, and re-Import w/ desired Duration set.
Otherwise, in Timeline View Mode/Expert Mode, you can first open the Info Panel, and then click+drag on the Head, or Tail of each Still, watching the Duration on the Info Panel, while dragging.