How to make my photos higher resolution with a Canon digital Rebel? Or a Nikon D3100?

I'm sending photos out to be printed. I have my Canon set to "large" and "high quality" in the settings. However, I'm warned not to print more than 20" x 30", the images are about 3000 x 2500 resolution. For this project, I'm looking to print something around 40" x 60" or 48" x 72"… So, clearly much larger. The ideal resolution for these photos is around 6000/7000.Am I doing something wrong? How can I get optimal resolution on either of these cameras? I assume the higher resolution will come from the Nikon, since it has almost double the amount of megapixels. If someone helps me out on this, ten points to best answer. I really can't figure it out -- especially since the Nikon is a camera I'm borrowing and am not too familiar with.

This is the Canon Digital Rebel model I have available to me (I believe it's a 300D EOS, 8 megapixels):

This is the Nikon Digital I have available to me (Nikon D3100, 14.2 megapixels):

Other information that may be helpful: I'm uploading them onto a Mac, into Iphoto (should I be uploading them somewhere else? Is this changing the resolution?) Can I change the resolution in Photoshop somehow? I do have the program.

Ten points and LOTS of gratitude to whoever helps out!

Yes, use the Nikon rather than the Canon, and be sure you have it set to its maximum resolution

also, you will get better results shooting in the "RAW" mode, and then processing/enhancing the RAW image.

You didn't say which Rebel model you have. The maximum mega pixels will produce' in theory' the best enlarged image. That assumes you use the full un-cropped image also.

The RAW mode on any DSLR will shoot at the maximum resolution, whatever it is, as will the "Large" setting for JPEGs. The quality setting doesn't affect resolution, but may determine what JPEG artifacts are visible.

As for printing, you should also consider how far away your subjects will be from the print. Anything handheld should be no less than, say, 150dpi. Posters and gallery work can be printed at lower resolution, even down to single-digit dpi if your audience is far enough away. A 3000×2500 image will print onto 48″×72″ at around 40dpi, so try making a small test print to see if the result is acceptable.

Resolution can be increased in Photoshop, at the cost of applying (destructive) filtering to the image. Image data can't be recovered, so Photoshop 'interpolates' (makes an educated guess) from the existing data.

I'm not sure if it helps, but here's a blog with decent info.http://www.digitalcamerastop.com/nikon-d3100-reviews-the-best-d3100-price/