Why can't I print larger size pictures?

When I try to print larger size photos I get a warning size that my pictures are not high res. I set my Nikon D40 to highest resolution available on my camera. Image quality - fine and image size - large.
What am I missing here. I know it has to be something. Please help!

Added (1). People thank you for answering my question. I need to be able to print 8x10. I understand RAW will give me better resolution but to tell the truth I have never used it and I have no idea how to convert files and don't even have a software for this. Any ideas?

You may want to try shooting in RAW and then import the processed RAW files as TIFF image files.

How large do you want to print your image files?

I have been able to print 16x10 inch prints from image files from a D100, a 6 mp camera that is much older technology than your D40

RAW files yield the highest quality. As fotoace stated, use a TIFF converted from a NEF(RAW) file. I can get 16x24+ straight from my S5pro which is technically a 6MP camera.

http://www.nikonusa.com/...wNX-2.html

This is the free software Nikon gives out with their DSLRs for raw conversion. Shoot RAW so your original file will remain untouched, then use the edit function to crop to 8x10.

An 8x10 is a very small print compared to what a D40 is capable of.

It seems that you are cropping in too closely. That is the only way I can think of that you would be getting a low res warning.