Hi Ayman,
Have you tried setting the image format to image/png?
You can change the format when adding/editing the External WMS in Spatial Manager

I have seen this white background when using image/jpg, JPG generally doesn't have an alpha channel (translucently) and should really only be used for aerial imagery or other complete coverage layers where there is a lot of colour variance which is well suited to jpg compression. PNG on the other hand is well suited to images with few colours or if you need translucency.
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Duri Bradshaw
Spatial IT Consultant
Insight GIS
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