Set a default still image duration, under OPTIONS | MEDIA tab, which will apply only to clips to be added to the bin.
For existing clips, select those you want, right-click on one and Change Clip Speed. Highlight the duration field. Type in a new duration. All selected clips will change.
That can be performed on bin or timeline clips.
There's also a feature available via the Menu (top-left corner of the window) | FILE drop-down named Add Images As Video that could prove useful. Experiment.