The container field can store your image and display it after you drag it onto the layout. You can insert pictures into this field, etc.
To minimize the size of the file you could store the pictures in a separate file and link to that file to display them, both on the IOS device.
Now drag the container field from the picture file onto your main file layout...
Curious as to why you use the Webviewer to display the picture when you can use a container field?
"store the pictures in a separate file and link to that file to display them, both on the IOS device." I don't think filemakergo local file store container field as a reference at all. You can test this by getastext on the container field and it wouldn't give you a file path but the image file name which happen when you stored image inside the container field.
I think it is a disadvantage than webviewer support isn't the same as in desktop due to ISO restriction. A simple example of that is I can have a layout using webviewer for image slider on desktop but that isn't possible in filemaker go if my image is stored in container or external folder within idevice.
I generally find filemaker handling of image take alot of CPU as it deployed quicktime. With webviewer the processing time is much better.