Has anyone developed an image tweaking tool similar to the Effects tools in Arcmap that allows the user to alter image contrast, brightness, transparencey, etc.?
I keep a close eye on things and I have not seen anything like it.
There might be something you could use in this collection of Dev sample widgets for Image Services,
GitHub - Esri/WAB-Image-Services-Widgets: WAB widgets specific to image services – WABIS
Hope this helps,