Problems with compositing

My equipment is I mac 2.5 ; Mountain Lion and iPhone 4s I stop motion3
I am unable to load a 4meg jpg as background and when I select the I stop motion library shots the chroma key stays locked at black.
I suppose what I really need is a ‘simple’ explanation of how to import a jpg background and then superimpose the foreground action, which was shot in front of a green screen

Can you provide the background file, so we can check it out?

Thank you for responding. This still was taken with my I phone Titus

I was able to use your image as a background. Anyways, I realized that there is a bug in the chorma key “Pick Color…” panel of the “Background”-tab: the left image should be the normal camera view. You should be able to select a color here. On the right side you will see the chromakeyed result. Did you setup your chromakey color here?

And in the “Advanced Setup” panel you have the chance to adjust additional values to fine tune the chroma keyer.

Thank you. I have now successfully imported my still to use as background. With that set I then rerecorded the animation in front of a green screen. I was able to select ‘pick colour’ and adjust in advanced settings. Some problems with lighting ,but I can sort that out. I am very happy with the result. However; How do I import a previously shot animation clip ( green screen) and then mix that with a background shot. many thanks

You can export the frames as an image sequence in the previously document and then import them into the new one.