APM Webtop Changing Search_Res image displaying unable to load on webtop

We are trying to change the Webtop "Find Resource" icon from the standard image to a font awesome search which has been converted to a png. When adding this to the "Full Webtop Settings" of the Branding section of the customization, the name changes too "/public/images/customization/Common/Dev_Webtop_customization/search_res_icon_en.png" but when viewing on the webtop, the following happens:

 

 

Any ideas why?

 

Thanks in advance.