Just click the “Mark as spoiler” option while sharing a picture and you are good to go.
On mobile, it is a bit trickier. What you need to do is rename the photo name so for example if the image name is “example.jpg” you need to make it “SPOILER_example.jpg”. Just add the “SPOILER_” in front of whatever the file name is and share that photo, it will be automatically spoilered.iPhone:Add your images to your files app. Rename your picture to SPOILER_[imagename] (SPOILER has to be all capital letters). Then attach the file to discord through your Files app, not your gallery or other apps.Android:Go to your file manager. Rename the picture to SPOILER_[imagename]. Now it is renamed you can share it on discord.