Display time for still pictures.

In 2025 you set the default Still Image Duration at Edit > Preferences > Timeline.   To apply a change, I think you have to delete all the stills from the Project Assets box and re-import them.   If you have a series of stills already on the timeline that have the same duration, you can select them all, right click and use Time Stretch to change the duration all at once for the group.   Display time can also be changed for a single image by grabbing the edge and dragging.

I have v2021 so your version may be different

In Expert Mode... Edit, Preferences, General, Still Image Default Duration