Hi,
this is logo image for header.
When we inspect we see that you set .jpg format, please place there .png image format (this also help your site to load faster :) ).
Except that, logo image must be small. So, as you can see on our theme demo page http://preview.magnior.com/jomelle/
we set size of image 206x21px.
Try that, if you still have same issue, please send us your WordPress credentials so we check what is going on.
Cheers
p.s. below you have private field which is visible only to you and us, so if you need to send some private data please place there, thank you.