Do you mean you have a video with a frame that you want to extend as a still picture? If so you need to put the timeline curser at the frame you want and go: File > export movie > as a still frame JPG. Then cut the video at this point (T key) and move the two pieces apart. Drag this picture / frame from the folder you exported it to and place it in the gap and drag as Asoeli says, to fill the gap.
Have you zoomed (in timeline mode) in to greater precision? The controls in the lower right can adjust the display as far a showing single frames. The following is from the Help file:
Also, in addition to asoeli's suggestions, you can do a right click on your image (again in timeline mode) and bring up a menu that includes 'Change photo length...'.
Yes a steep learning curve, but I assure you well worth the effort!
Here's a screen shot of two pictures on my MEP timeline zoomed to single frame resolution:
Note that the time marker above the top one is showing 00:00:00:01 (hr:min:sec:frame).and the right click menu is showing the bottom image as have a play time of 00:00:06 (min:sec:frames despite the 'sec' label!). I've also typed in the best I can to represent the cursor (which is not screen grabbed) when it is available for stretching the top photo.
If this isn't similar to what you are seeing can you post an image please?
I'm still not clear what you mean by '...a frame that is a picture...' ? Is this a video clip and you want to freeze it at some point (typically the start or end of the video clip) and insert that still image for a certain duration?
If so, get the required frame displayed in the preview monitor by moving through the timeline by whatever method suits you, with final precise navigation using the left and right arrow keys. Then simply export it:
Then import the image immediately to the right of the original frame. Finally adjust its length to suit.
Set standard length to pictures:
Go to Program Settings (shortcut Y) , tab Video/Audio. Here you can set the duration you want. (is your set to 1 frame?)
Comment to Terry, it seems that you have changed all the shortcuts, as they do not correspond to my MEP 2014 Premium !
regards A