One option would be to disable inline output for R Markdown documents. This way, instead of appearing in your document, the plot will be displayed in the Plots pane.
You can disable inline output by going to Tools -> Global Options... -> R Markdown and unchecking the box that says "Show output inline for all R Markdown documents". This will disable the inline plots generated by code chunks, like the one in screenshot above. Note that you may have to close and re-open RStudio for this new setting to apply.
If you also want to disable previewing of imported images (i.e. via the Markdown syntax , then in the same menu you can change the setting for "Show equation and image previews" from "Inline" to either "In a popup" (the image will be displayed in a thumbnail if you hover your mouse over the that line) or "Never".
Here's a screenshot of R Markdown Global Settings after disabling inline output from code chunks and displaying imported images in a popup: