We ask for images to be saved at 300dpi or greater for best results.
Most software programs open at 72dpi. You'll want to save the image, in your design software, at 300 dpi or greater BEFORE uploading the finished PDF to the project center.
Images supplied at less than 300dpi may appear "grainy" or "pixelated". This is especially apparent on photographs.
If the image is already at the intended output size and is lower than the recommended 300dpi, changing the resolution isn't going to improve the image quality dramatically. Once it's saved as a lower resolution at the output size, the damage is done – the pixels are what they are. Any increase in resolution simply re-samples existing pixels, blurring the image at best, not improving quality.
I'm afraid there is nothing else you can do to correct the resolution short of starting over from scratch at 300dpi at 100% output size.