It's not clear what you are asking, Hector.
What model of camcorder is your video coming from and how did you get it into your computer? If you are using photos, have you ensured that your photos are no larger than 1000x750 pixels in size?
What do you mean by "when I want to record my project in a littler resolution (800*600)"? How are you changing these settings?
Finally, how to you plan to distribute your final video? Online? On DVD or BluRay? There may be a more efficient way to build and edit your project than the method you are using.
For what I am reading, per your desires, I would match the Project's Preset to that of your Video, in every respect.
Scale your still images to match the chosen Frame Size. This ARTICLE will give you tips and more detail.
Now, to get the Video to "come out of the stills," be sure to turn Scale to Frame Size OFF. Then, you will do a PiP (Picture in Picture), with the stills on Video Track 1, and the Video on Video Track 2, above the stills. You will then use the fixed Effect>Motion>Scale and Keyframe the Scale of the Video. You will start with a Scale of 0, so that the Video is not visible, then Keyframe the Scale to what you need, whether PiP (smaller than the stills), or full-frame. If you want the Video to "disappear" back into the still, just reverse the Keyframes.
When done, you can Export/Share to a smaller set of dimensions, with most formats/CODEC's. You will want to keep the Aspect Ratio the same, as with your Project's Preset, so a little math might be required.
If this is not what you want, please correct where I went wrong, and good luck,