Make an image background fully transparent in LibreOffice impress

I am preparing a ppt in LibreOffice Impress (by giving/setting a colour for side theme ), and i have pasted an image, So now i want the background of the image to be transparent. I even tried with options - Image - colour mode - watermark… but still the background of the image is not fully transparent (such that the slide colour is visible, ie., the slide colour should be visible through the image…). So, how to solve this issue…?
Thank you

Do you expect good results with .ppt? Why not .odp?

Impress Is not a tool for editing raster images. If you need an image with transparency, you need to prepare that in a suitable editor, such as Gimp. Apply the required transparency there to whatever you need (color, or areas, or make something semi-transparent), and then use it in Impress.

I don’t think you can make a directly inserted image transparent. (Don’t know anyway what the “background of an image” should be.)
But: You can use a shape (rectangle e.g.) as a kind of frame for an image. Go to the ‘Area’ tab and choose ‘Bitmap’. Now you can import images of any format. Choose the additional properties and go to the tab '‘Transparency’…
I would expect rather ugly results.

Look at Add or remove transparency | Free Online Image Editor