A white bar can be introduced by cropping an image but, instead of making it smaller by cropping it, stretch the crop_grab handles and make it larger. The added area is white.
That being said there is something horrible about the image as it is a highly magnified tiny portion of the real image and I found it tricky to force it back to its original size with the entire image displayed. I "un-cropped" it till it was much bigger and included most of the image. I could then remove the white bar.
See
[Tutorial] Some useful hints on using images for a discussion on how best to handle images in Writer. In particular,
It is far better to process your images in an image editor so that they are exactly how you want them to appear before inserting them into AOO.
Also, don't
paste a photo-like image in - it gets saved as a huge PNG file. Instead insert a JPG. Or don't save a photo-like image as a PNG - save it as a JPG.
If you Copy a photo to the Windows Clipboard and Paste it into Writer, Writer does not know that the image on the clipboard is a photo. Writer therefore (correctly) saves the image as a PNG file inside the .odt document so as to guarantee that no quality is lost. Photos saved as PNG files are likely to be MUCH larger - 3x or 4x larger is typical - than the same photo saved as a JPG file.