When assigning images to DocMart items you will notice three buttons, "Use File", "Use Thumb File", and "Use Both".
1. Use File: Inserts the actual file into the proof, useful for putting larger images or PDFs on DocMart Items
2. Use Thumb File: Inserts the MyOrderDesk thumbnail generated by your file library settings, this is a .PNG file.
3. Use Both: Inserts the File into the Proof section and the Thumb file into the Thumbnail section.
In most cases hitting "Use both" will save you time and clicks when creating these DocMart items.