Hi @BerkayAkar and welcome!
Unfortunately I don’t think there is currently a way to add images directly to workflows.
I have wanted to do this myself as I think it would be a useful mechanism for providing documentation and I am quite a visual person myself so often like to document using diagrams.
Looking at the following from April '20, it has been raised I believe as a “feature request” Adding an image to my knime platform process?
In the meantime, one suggestion I have which may be of use (although not specifically what you want) is that if you have any diagrams relating to the workflow, and you are able to make them available on a shared webserver, for example, then you could add a link to the image address in the workflow documentation (description) panel.
Then at least if you click on the workflow background, you can then get quick access to diagrams (via the “Description” panel, if you leave it visible), even if they aren’t visible directly in the workflow