The easiest way is to set the Duration of Stills from the Edit>Preferences Toolbar Menu. Set it to what you want, and then Import the still images.
It sounds like Hunt answered your question right on, but, on second read, I was wondering if you were asking how to find the Time Stretch Tool.
If so, right click the clip on the Timeline and select Time Stretch to get the Time Stretch Dialog. For a rougher way, click on the Time Stretch icon above the Timeline and use your mouse cursor to lengthen or shorten the clip duration. But that will only get you one clip at a time.
On the other hand, there is always the Create Slideshow feature accessed from right clicking highlighted media in the Project Media View. That will get you image duration for more than one image at a time. Lots of possibilities. Perhaps I have read more into your question than you asked. If you need any further information, please let us know.
Just to tag onto what Tony and Hunt have said, note that Time Stretch won't affect photos. But Tony's suggestion to use the Create Slideshow function is very good.
Also note that Hunt's suggestion to change the still length in the preferences will also work. However, this preference change will ONLY AFFECT STILLS BROUGHT INTO YOUR PROJECT AFTER YOU CHANGE THIS PREFERENCE. In other words, if you change this preference after you've added your stills, they'll still be at your default 5 seconds. Stills added after you change the preference will come in at the new duration.
Also, don't forget to optimize your photos to no larger than 1000x750 pixels, per the FAQs to the right of this forum.
Just an add on that may just be my reading of what you wrote.
"note that Time Stretch won't affect photos"
Did you mean that as a generalization or specific to this Premiere Elements 4 user's need to get the same still duration for multiple stills at the same time?
In Premiere Elements 4 and 7, I have observed that you can change the still duration by right clicking the still image and selecting Time Stretch from the pop up menu and setting the Duration as wanted. One At A Time Basis since highlighting/selecting more than one still image at a time results in a grayed out Time Stretch in the pop menu. AND this works for shortening still image duration and has its limitations when increasing the duration for images other than the end or where there is a space in the Timeline to accomodate the increase.
However, the great perk about Premiere Elements 8 is that you can select ALL your still images on the Timeline, right click anywhere in the highlight, select Time Stretch in the pop up menu, set your duration and ALL the still images will end up with the same still image duration on the Timeline.
So based on all that, I am thinking that I misread what you wrote, initially thinking that is was a generalization rather than specific to the Premiere Elements 4 user's question???
Nope. I stand corrected, Tony. You certainly can change still duration on your timeline with the Time Stretch right-click tool. Thanks for clarifying this for me.