Hi.
I recently upgrade my dotnetnuke from 4.9 to 5.1.2 and the icons in the solpartmenu do not display correctly same as image you post.
I correct the problem executing the follow two sentences in database: (you could use the query tool in dotnetnuke host option)
But this could no be compatible with DNNNavMenuProvider, only with SOLPART
----------------------------------------------
update dbo.dnn_Tabs set iconfile = replace(iconfile,'~/images/','')
where iconfile like '~/images/%'
----------------------------------------------
Israel Garcia Anido
www.nearsolutions.net