Hi Rawand,
What version of Survey123 Connect and Field App are you using? also what version of Portal?
Also, what size is the png file, and can you share a copy of the xlsx form and png so we can test on our side.
Phil.
I tried with another .pngs it works but not working with this png its really weird when i publish it the thumbnail will disappear, please try in your side
i attached png and XLSForm.
thank you