Imagine I want to build something like Instagram profile pages, which show a gallery of pictures, and upon long-pressing, will zoom-in, focus on the picture, and display some secondary information (like a caption). How can I communicate to first time users that this action exists?
You can add a toast notification when launching the app that fade out after a few seconds.
Original image from dribbble
You can use an indicator for touch and wait. As soon as the user touch any image from your gallery start an amination something like the below image. The animation should be the short may be for 2-3 seconds.
This will indicate the user to wait for few seconds as soon as she/he touch any image.
Hope that helps.
I would go with showing semi transparent black layer on first image, and in the middle icon and text "Press long for more info" https://thenounproject.com/search/?q=long+press&i=1817631
After user has first time sucessfully did the interaction, it should be fine to remove the layer and the explanation.
This kind of interaction, which is not obvious, is the type of information that should be included in the onboarding process when the user first login.
This is what Tinder does to explain to users how to swipe right and left, and the consequences of both actions.
Also, micro-interactions like something loading while the user press might be useful for the user to understand that this interaction is different. Think about how you can take both photos and videos with IG stories with the same button. They indicate that you are recording with the loading line around the button while pressing it.
Hope this have helped you :)