Yes, you were right.
The photo was not png and we turned it into a PNG but there were two problems. First, it has no transparency it looked like a simple JPG image. and the second problem is that we use the Background video in home page which also lost its accessibility and became motionless like a picture

Hello Mary,

are you sure the photo you uploaded is png?
and the name is MyLogo ?

I also forgot to tell you how to upload the logo to the path YOURTHEME\images

try again and tell me

Hello. Thnak you Rafail. I tried to do that but we got a picture like this.

Hello Mary,

Go to Path : \YOURTHEME\plugins\default\theme\page\elements\topbar.php

AND CHANGE LINE CODE "17"

<span><a href="<?php echo ossn_site_url();?>"><img src="<?php echo ossn_theme_url(); ?>images/MyLogo.png"/></a></span>

Then upload the image with name MyLogo.png like the code I gave you
The height dimension of up to 40px or 30px depending on the theme used

Its very simple go to "ossn/themes/goblue/images/" and replace logo.png thats it and if u wants to change background then simply replace background.jpg