On the toolbar at the top of the text box is three vertical dots, if you hover your cursor over it it'll say "more options". Click that and it'll drop down a couple more tools, select the one that looks like an eye with a line through it, it says "spoiler" if you hover your mouse over it. Click that. enter the title of the spoiler when the box pops up, then insert your image into the middle of the two brackets that say
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EDIT: Added the image, you can also just manually type out the brackets as they're shown in the image, anything within those brackets will be under the spoiler tag.