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Do you have an example where it does not work?

In general:
* if you upload a JPG, where the pedal is placed on an evenly background, the software tries to autocrop the pedal and change the even background into transparency.
* if you upload a PNG there is no auto image processing involved.

So if the autocrop does not work uploaded PNGs that were edited with an external software indeed give the best results.

Also check where you get the dimensions from.
Instead of giving the right dimensions of a pedal, shops like Amazon often just list the dimension of the package.