Arcpad use a very old plugin to load ecw images. The version is the old 1.7.0.424.
It still works on arcpad v10.2.2 with arcgis v10.2.2 and Win7 X64 SP1
But you cannot load the last version 3 and 5 of ecw format.
You need to change/save your image from ecw to jpg2000 or geotiff
You need at least to upgrade to win8.1 and download the plugin from the intergraph website