Hello Walter Wedler,
Thanks for asking your question here in the Apple Support Communities. I understand you wish to keep the last frame as a still, or possibly a graphic so there is no blank frame at the end of your imported movies. I believe there are a couple of options you can choose from to do this. You can read over these suggestions, and determine which suggestion you like best.
I believe the best option is to shorten the clip at the end, removing any black screen so when the movie ends, it will end on your last frame or photo. You can do this at the specific frame at a time, without using the trimmer: Move the pointer to the end of the clip and hold down the Option key while pressing the left or right arrow key. Click Play in the trimmer to preview your efforts and click Done when you're satisfied with the results. There is an Apple help article here: iMovie 11: Adjust the duration of a photo, still frame, or background clip to offer another method assist you with this also.
There is another feature called hold last frame that may also do what you are looking for. It is discussed in this article from Apple: iMovie 11: Add interesting visual treatments to the ends of video clips.
Since you are new to editing, I thought you might also like to see all the special effects you can apply to video and photos shown in this help article:
iMovie 11: Types of special effects you can apply to video and photos.
Happy editing! Thanks and have a great rest of your day.