The duration of a JPEG image, or any other video clip, can be controlled in several ways.
As explained in the
Video Edit Magic help system, you can select the clip, then drag the top-right corner of the it to change the length. you can also select it and click the
Resize Sources button on the Timeline, then change the end time. This second technique lets you set exact durations in seconds. (Follow the link for an illustration.)
You may be having trouble because the drag-to-resize technique doesn't work to make clips longer unless there is open space to the right of the clip to expand it into.
I hope that's helpful.
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