White box or transparent icon? Find the difference
A white background may be part of the image, part of the preview or part of the destination interface. Changing the filename to .ico cannot tell these cases apart.
Check the source before converting
Open the source against both a dark and a light background in an image editor. JPEG has no alpha channel; saving a JPEG as PNG does not remove its white pixels. A checkerboard baked into a screenshot is also image content, not transparency. Remove the background in an editor and export a real transparent PNG.
Look closely at the edge
A pale fringe can remain when an image was cut out against white. Inspect the edge on a dark background and correct those pixels in the source. IcoJet preserves available transparency and adds transparent padding to fit non-square artwork; it does not erase backgrounds, repair halos or crop a wide logo into a square symbol.
Test where the icon will be used
Convert the PNG and check the result against more than one background in an icon viewer and in the destination application. If the file is transparent but only one app shows a box, inspect that app’s icon presentation and asset requirements. Keep a little breathing room around the artwork, but avoid so much empty margin that the symbol becomes tiny.