The window image displays in Impress the same way is displays when viewed so
prepare your image in an image editor to look as you want it before inserting it into Impress. If you just want the curtauins and window frame, crop the image. Set the edges to be white; or set them to be transparent and white background will then shine through.
The only change possible to make the "glass" transparent in which case whatever is behind the window image shows through the transparent areas.
1. Set the colour of the window image "glass" to be, say, blue.
2. Save the image as a PNG file, saving with transparency. Set blue to be transparent.
3. Insert window image.
The window image "glass" now shows
whatever colour is behind the image.

- Clipboard01.png (7.16 KiB) Viewed 2044 times
test.odp
- (40.6 KiB) Downloaded 241 times
See
[Tutorial] Some useful hints on using images for a discussion on how best to handle images - and see the Impress Manual.
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