Tell us something about these photos that you brought to the printer, specifically the size in pixels (width and height).
Perhaps you are trying to print them at an extremely large size? What size in inches or centimeters (width and height) did you want to print them at?
Perhaps you changed the size in the Export dialog box, like you checked the "Resize To Fit" box and entered some numbers there? What did you do? Can you show us the Image Sizing section of the Export dialog box?
I checked the export dialog box and had checked the export resize to fit dialog box and changed it 4x6". The resolution in that box now says 72 pixels per inch. What should this setting be?
If you are specifying 4inches x 6inches, set the resolution to 300
Your answer has raised another question. Is there a guide somewhere, as to what to set the resolution number to for various enlargement sizes?