To export an Arcgis Online view in JPG or PNG

[CelineCouvez]

To have an export map option like it is available in Desktop :
Export the current view in a format like JPG or PNG, in a high resolution (more than 96 dpi).

[pepe0001]

Esri does have a solution for this already. It's called MapStudio. 

[SteveCole]

2016 update: "Esri MapStudio is being retired and replaced by ArcGIS Maps for Adobe Creative Cloud."

And, frankly, that's not a real solution to what the OP was suggesting. If you're just using AGOL, the export map option is still a valid feature request.