On Resolution:
Whether you upload a 2400x3000 image (portrait) or a 3000x2400 image (landscape), Mpix will orient the image to fit on 8x10 paper. So that first time you uploaded, you either didn’t upload a 2400x3000 image, or you selected some other paper size besides 8x10.
On Cropping:
When you first open the crop page, the crop lines should be along the edges, which will print the entire image. Nothing will be cropped. If you want to print the entire image then don’t touch the crop tool.
On Exact Sizing:
First, read what Mpix says about exact sizing in the FAQ:
Exact sizing?
Please note: Digital printers have what is called, over sizing. Over sizing is a process in which the image being developed onto the photo paper is magnified by a certain percentage to counteract paper shift within the printer. Photographic paper is loaded into the printers in rolls. As the paper travels from the roll through the machine it can drift up to 1/8 of an inch by the time it reaches the lasers that expose the paper with the image. No amount of calibration on the paper path can prevent this drift. A 1/8 of an inch is about half the radius of the pen or pencil on your desk. The over sizing that is applied to each image runs between 1.5% -1.7%; we generally quote the percentage at 2% for a round number to work with. How can you use this information? Well the over sizing is so minimal, that 99% of your orders will not be effected by it. However, in certain instance where you may have images butted up against the outer edge like in a collage, or a pin stripe around an image's perimeter, you will want to take this 2% value into exception. In most cases you can do very simple math to calculate the expected over sizing. Our message is to add the 2% additional space to the perimeter of any potentially affected image. For regular prints such as an 8x10, the additional leeway your need to provide is a 1/16 of an inch in the 8-inch dimension and 1/5 of an inch in the 10-inch dimension. Of course, the larger the print dimensions, the more image space effected by over sizing, thus the more padding that is needed.
In plain English, this means that Mpix can’t put your perfect 8x10 image on an 8x10 piece of paper because of mechanical tolerances. Experience tells Mpix that the paper can drift 1/8th of an inch while rolling through the machine. If they print a perfect 8x10 image and the paper happens to shift by 1/8th of an inch, you will get a 1/16th inch white strip on the edge of your photo. To avoid this, Mpix will automatically increase the size of your image by 1/8th of an inch. They’re putting a big picture on a little piece of paper so there can be no white strip.
How to use this information? Think of it this way...Mpix is going to crop your image by 1/16th to 1/10th of an inch on each border no matter what. If every single pixel of your image is important and must be printed, then add a small border to your image.
On DPI:
From the FAQ:
What DPI is optimal?
250 dpi is recommended.
Why 250? Well, first off this 250 actually refers to PPI (pixels per inch.) DPI is more accurately use to refer to a printer (such as a 600 DPI printer.) So again, why 250? Because high-end production printers print at 254 PPI (effectively 1200 DPI) so 250 is a nice round number that comes very close to what the printer is actually doing.
As you know, Mpix will increase the size of your image. However, you don’t have more pixels to cover the increase. Mpix will simply spreads the pixels you do have across the distance required. So your 3000 pixels are being spread across 10.2 inches, for an actual PPI of 294.1. You’ll never get images printed at an exact PPI count. Or, more accurately stated, you’ll never get a print where one image pixel equals one printer pixel because Mpix is always going to stretch whatever you send them. All this is done by the printer, which automatically scales your image.
I wouldn't worry about DPI and PPI. As long as you have at least 200 PPI your image should look great. The DPI setting in your image is practically meaningless. It can say "10" or "500" and it won't affect anything. The easiest thing to do is to upload your images, crop your image using the Mpix crop tool, and crop a bit on the large side to account for the 1/8th inch you're going to lose. Only in rare cases where you need every single pixel to the edge of an image should you do extra work beforehand, such as cropping and adding a small border.
And when cropping don't forget to account for your mat!
Hope this helps!
As always, corrections and opinion are appreciated.