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You can drag and drop image file in that window or you can click image button which is in the left sidebar the one just before last button with chain.

P. S. But looks like you are using outdated version. It has a bug with image uploading. Update to 1.0.1 for a fix.

Allright, thank you @dunsky

Copy the picture and paste it there @saiful.nizam

Thank you Sir.🤗

Just tap on the image icons there. It pretty easy, just look carefully or rather, zoom in.. The new improvement does that 😀