I'd say it's Archival. You can do your test there, and delete the content once you’ve finished. I believe you cannot trash your thread in Archival, but you can leave the content blank. The board is full of test threads if you take a look now (some arte test Ann threads too).

This thread seems pretty informative on image placement and sizing: Guideline on posting images (with size adjustments), hyperlinks

The actual images you link should be less than 2,5 Mb.