hello,
here they say they have both public and less public...
http://doc.arcgis.com/en/arcgis-online/reference/terms-of-use.htm
for example you can make a request like that without having any registraton
https://server.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/export
?bbox=15424380.8117,-4158174.33871,15429272.7815,-4153282.3689&bboxSR=&layers=&laye
rDefs=&size=409,409&imageSR=&format=png&transparent=false&dpi=&time=&layerTimeOpt
ions=&dynamicLayers=&gdbVersion=&mapScale=&f=pjson
there is also a gui
https://services.arcgisonline.com/arcgis/rest/services/World_Imagery/MapServer/export?bbox=-2.7680880158351306E7,-195164.8424795773,2.7680880158351306E7,1.9971868880408563E7
hope it can help
best stefano