Is there an SDK that supports run time image target creation for Unity?
I have tried User Defined Target from Vuforia but it does not give me the option to save the image for future use.
8th Wall SDK has image target support. You can provide an image at runtime by simply provide the RGB pixels to the engine configure call.
8th Wall calls into your phone available native support. So if you have ARKit, it uses ARKit image detection. If you have ARCore, it will call into ARCore (arcore 1.2 has image detection support but it just got released so it will take the 8th wall team a bit of time to roll out its support). The nice thing is you write your code once and it will just work.
Note: I worked on this product as my day job and was involved with this specific feature. Feel free to add comment to this answer if you would like more info.