I use a workaround for such tasks.
You can use OBS hotkeys, or any external program that can issue commands to OBS, like websocket, UpDeck, or TouchPortal.
OBS can show and hide sources on a hotkey. It can switch scenes with a hotkey. And you can set a default transition for a scene-- it sounds like want you want is a fade.
So I make a scene. Call it "graphics". Put in it the images you want to have fade-in fade-outs. Then set them all to Hidden.
Now take the main scene in your collection and duplicate it. Call it "main with graphics" or something.
TO that duplicate scene, add as a source the Graphics scene you made above, and set the default transition for "Main with graphics" to be Fade.
For the next step, you use an automation program or macro program like UpDeck or TouchPortal (I use TouchPortal).
Assign the following actions to a macro:
1) Unhide a specific image
2) Transition from Main to Main with Graphics scene
3) wait X seconds
4) Transition from Main with Graphics to Main
5) Hide image
This is not an ideal procedure but it will allow for fadeins of images.