you can crop it to that ratio in software (capture, photoshop, etc.). there will be a menu or tool somewhere that will allow you to select the dimensions and resolution. for the dimensions you obviously want to pick 8x10 inches (you'll have options for pixels and other units). for resolution somethhing around 300ppi should work well.
then process as you desire and convert to 8 bit. if you know what color space their printer uses then you should convert to that (or shoot in it to begin with). even better would be if they could give you the profile for the printer.
Just one more question. I opened my pic in PS, went to Image>Image Size and the size is Height 10.027, Width 6.667. Now, in order to print to 8x10 I'd have to stretch the width by 1.3 inches, right? This picture was taken at the highest resolution with my D50 so the pixel size is 3008x2000.
Now, I'm a bit more confused, lol. I must be making this way too difficult.
given the numbers you supplied, i'd say that the 10.027 and 6.667 are in units of inches (3008/300 = 10.03 = , 2000/300 = 6.67). so your aspect ratio is the usual 2:3, not 4:5. but the resolution (300) appaers to be good for printing.
if i understand you correctly i wouldn't try to stretch the image as that will lead to distortion. you can either crop it to 8x10 (2400 x 3000, 4:5 aspect ratio) or interpolate up to 8x12 (2400 x 3600, 2:3 aspect ratio).
this is relatively easily done in your software. what software are you using?
i don't have photoshop 7, but if it's like the versions i have you first select the crop tool. then in the boxes you would enter "8in", "10in", and "300ppi". then make your selection with the crop tool. you can then reposition it by using the arrows on your keyboard if it's not exactly the area you want, and you can grab the crop frame and drag it with your mouse to make it bigger or smaller. when it's as you wish just hit the enter key. photoshop will interpolate the area you crop (no matter how big or how small) to 8x10 at 300. if you want to specify an interpolation method you can try "bicubic" or "bicubic smoother" for creating larger images. "bicubic sharper" works a bit better when creating smaller images. the names may not be quite the same in your version.
when you do this you wull crop off part of the image since you're cropping to 4:5 aspect ratio and the original image is 2:3, but at least you get to choose what to crop.
Yes, you should be able to preserve any aspect ration by selecting a standard size canvas. If you go to the image size menu in Photoshop you can select either image or canvas. If you have, say a 4x4 image, you can select a canvas size of 4x6. Just order a 4x6 print and you should get the whole 4x4 image bounded by 1" of white in the long dimension. I hope this is clear. I don't print at Walmart, but I print at Costco and this method works for me.