Controlling image opacity with a slider seems to be a fairly common question, so I created a script that lets you control the opacity of images, color corrections, and materials.
You can find the script and tutorial here. It goes over how the script works, but if you aren't a coder you can just copy and paste the script. All you need is a UI slider and a target image and you are good to go.
I think the video is a little rambly, but I do spend a bit of time going over how the script itself works.