Like we wrote above, images id's you can find in Media section. If you hover over the image, you will see the link of the image in the bottom where ID in the link of each image is.
No, you are doing it wrong. We mean MEDIA section in main WP Dashboard for the whole time. Just go to MEDIA section in main dashboard on back-end where all media are. This is the only place where you will find ID's for images. Just take a look at https://snag.gy/O0fpxn.jpg
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portfolio items can be edited under Theme options > Portfolio section.