Using a background image on a header/footer on Libreoffice Calc

I am trying to create a header and footer with an image in the background. I am not using any text in the header/footer, only the image.
The images are png files with transparent background. Their print size is around 26,5cm by 3cm. In pixels they are around 3130x363. The orientation of the page is landscape.
I am using version
I have setup the header and the footer through the Format → Page Style… menu selection. On the Header / Footer tab I choose the Background tab and then Image button. I Add/Import the image and then under Options, Style I choose Custom position/size. Under Size I set the above stated Width and Height. Under Position I choose Top Center. Then I press OK.
The image appears in the header / footer oversize. When I open the above dialog again the Width is reported 82.82cm and the Height 6.59cm.
Can anybody explain the way Calc uses images in the header / footer? I have read that the image must be pre-sized because it can not be re-dimensioned. That is why I provide an image with the exact dimensions I want but apparently some sort of re-dimensioning occurs.
I tried scaling down by 30% but that does not work in Calc as changing the Width and Height value also fails.

Edit: I found a solution that works but I consider it a hack.
1 .I changed the Style under Options from Custom Position/size to Stretched and clicked OK. 2. In the window Page Style, in the Header or Footer tab I set the Height to the height of the image.
3. In the Page tab I set the Left and Right Margins to (Page Format Width - Image Width)/2
This way the image appears correctly proportioned. Of course this works only for images that have been prepared to fit the page area of the paper used.

So the question remains. Does the Custom Position/size Option works? Can somebody explain how it works?

Can you upload your picture here?

Does it make any difference? I do not have the permission of the creator, that is why I gave all the details above.
If it is necessary I will ask for permission or find a similar one.

These are the image properties of the header and footer images.

This seems to be a bug.
Just select the Brick Wall image. Select Custom position/size, and set Width to 5 and Height to 2. OK
Reopen the dialog, and values are changed.

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LibreOffice makes it to fit into the width of page margins and the height of the header/footer.

In that case,what is the purpose of Custom Position/size if the user cannot set the Width and Height of the image?
Is this a feature not implemented in the right way or is it a bug as mentioned in the previous reply?

In these cases you can upload a prepared image with same size (physically and pixel size), same file type; and a sample ODF file. It is important for the helping.Then we will able to trying to place, position and resize (strech) the “actual image” in the actual sized Header/Footer.

Try to use optimized image. Crop it (or complete it with white or transparent area), resize/resample it for the available place of the Footer/Header. Use third party image manager/editor software.