Is there a way to upload a filter using a custom image background?

Yes, in FF3 there are more preview images, not just the life saver.

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crispalomino wrote: Is there a way to upload a filter using a custom image background?

If you could explain in more detail your question and give more information it could help much to answer you better.

I suposse that you are talking about submitting filters to the online FF library and that it will be able to show OTHER different preset previews than ONLY having the original lifesaver.

If this is what you mean, the answer is as said above, is YES, ONLY FF 3.0 has the feature and ability to use multiple built-in images and you can submit the filter with different preset images.

As said by CFandM you can ONLY use the already built-in images in FF 3.0 and can´t use your own, because they will appear right in YOUR computer BUT when you submit the filter with your own images preset they will give an error as in the server of FF your own images are NOT there, and you can´t upload your own preview images to the server.