How did you change the dpi in Preview?
When you adjust the dpi, make sure the pixel size remains the same.
Set the "Fit into" pop-up to "Custom" pixels, and enter the pixels your photo currently has, but change the dpi to 300.
Your photo will now have smaller dimensions in inches or cm, but still have the same number of pixels and a higher resolution tag.
Yes you can do tat but it akes NO DIFFERENCE - the newspapr has no idea how digital photography works
see http://www.rideau-info.com/photos/mythdpi.html for more details
Simply put no digital file has any DPI (yes there is a DPI field in the EXIF data but it is simply a number and any competent printer simply ignores is since it is meaningless
DPI is dots per inch and therefore can not exist until you have "Inches" - until you print - then DPI is a simple mathmatical calbulation - the dots (pixels) divided by inches (Dots Per Inch)
If you give the newspaper a 3872x2592 file then they can print it up to 12.9066667 Inches x 8.64 inches at 300 DPI (3872/300 x 2592/300) -- if the print is suppose to be larger than that then you need a higher resolution camera
And although it makes no difference newspapers do not need more than about 120 DPI because of the overall quality of their printing
Thank you so much for the quick reply. We did as you suggested up to a certain point. Changing the dpi gave us an enormous size so we reduced it probably by too much. They replied "We appreciate the effort, but with Photoshop changing the DPI reading actually decreases the image quality as the pixels appear blown." We have redone the image in Preview and kept the size to the original and are waiting to see if this works for them.