Ah, there are several things that can go wrong here.
Where are your pictures stored?
What is the code you’re using to display them?
The images need to be online so other people can access them. Itch can host them for you, if you want.
Here’s a sort of example: if you’re make a zip file to upload to itch, its contents might be something like this
index.html -- this is your exported game
MyImage.png
-- you could also put images in a sub-folder, if you like:
pics/AnotherImage.png
Then in your twine you can use [img[MyImage.png]] or [img[pics/AnotherImage.png]] to display them.
There are a couple of tricky bits here: the capitalization matters. If you get it wrong, Windows won’t care and it will still work, but then it will fail when you upload it to Itch (or any other web hosting, probably).
If it’s in a sub-folder, you want to use forward slashes (pics/AnotherImage.png instead of backslashes pics\AnotherImage.png). It will probably still work if you get this wrong, but better to be safe.
Also, a useful tool for diagnosing all sorts of problems is the error console in the browser: that’s F12 on most browsers, I believe. There will be multiple tabs on the pane/window that opens, the “Console” will let you see scripting errors, while the “Elements” will let you see the HTML that Twine is generating.
If you right-click on the broken image and choose Inspect, it will open up the Elements pane with the <img> element selected. You might be able to see from there what’s wrong, or edit the URL for the image to experiment?
For future reference, the official Twine Cookbook contains a section that describes Publishing Twine Games on Itch.io. And while the image associated with Step 3 only shows a sound “child” folder the instructions will also work if there are any other “child” folders, like image and/or videos.