There's a few ways you can do this,and of course it depends on where/how you want to store the images. An important key here is that there are two ways to display images, using an Image URL (passed into the Image widget), or using an actual Image basetype (passed into a Value Display widget).
Probably the simplest way would be to add a property to your device Things of baseType image. Then you can simply grab it and display it using the Value Display widget. The image widget does not accept this type of data, it takes an ImageURL, so you must use the Value Display widget
Alternatively, if you are set on holding the images in a file respository, you can write your own service that dynamically builds the path input for the repo's 'LoadImage' service based on some input, probably your selected device's thingName. I forget if this returns an Image or an ImageURL, but either way you can display it, you just need to use the appropriate widget.
You can also make a bunch of media entities and store the images in there, then build a service to grab the image from them or have a service build the REST url location for them.
Surely there are more ways depending on what your use case is, but you can experiment with these and see if it helps.