In the Preference/General you will find Stills Image Default Duration.
After setting the duration you need to re-import.
You can also change duration for stills in Clip - Speed/Duration, just select all the stills.
If the stills are an animation sequence exported from a 3D program or something and are sequentially numbered 0001.tif, 0002.tif for instance, you can go to the Import dialog, select the first still, then check the box for "Numbered Stills" and this will import the whole image sequence as a single "clip" then.
Safe Harbor Computers
..another way is to use After Effects to create an intermediate clip from the sequential files.
I do this for the convenience of the single movie clip and I rightly or wrongly believe ....it is easier on Premiere to not deal with a mutlitude of clips in the timeline and system.