Welcome to the forum Florpet,
Do you mean setting the Default Duration of Imported Still Images, say to create stop-motin, or time-lapse?
If so, then go to Edit>Preferences>General, and set the Default Still Duration, as required. Note: that will also set the Default Duration for Titles, so keep that in mind, if you do add any Titles. You will likely want to reset that Duration to longer, when you get to the Titles.
This shows the dialog panel: