Black lines around picturers after pdf export

Where can I find this in InDesign 2018?

In case my post wasn’t clear the lines are an Acrobat display artifact, they won’t print as long as the output is from Acrobat to a high resolution printer (i.e., any offset press). Acrobat’s Page Display Preferences let you choose how a high resolution print PDF will display on a low resolution screen—print resolution is typically 2400dpi or more, while screen resolutions are typically 100-300ppi.

When you flatten transparency (PDF/X-1a) the page is sliced into sections, and the Acrobat display might anti-alias the section edges depending on your preference. With high resolution print output there is no need for anti-aliasing, so the lines will not print.

In any case you should still get a contract print proof before going to press.

As rob-day pointed out, the stitching line are caused by your PDF export settings. If you have the option to use the Press Ready PDF export setting (which doesn't require flattening) rather PDFx1a, that should solve the problem. At least it worked for me.

Try unchecking Smooth Line Art in Acrobat’s Page Display Preference, that should prevent the flattened areas from anti-aliasing on screen.

Check to see if an object style is applied by selecting an mage and checking the Object Styles panel; if so, remove the stroke setting.