DR Jennifer - Those photo sites allow you to preview the photos before you order them.
As you indicated, the proportions between 3-1/2x5, 4x6 or even 8x10 are not equal. As I mentioned, even when you take a picture with regular film, you never get the "whole" picture, the whole frame (of the negative). Most shots are "framed" in the center of the negative, and depending upon what size the final print is, sometimes you get more or less of the outer edges of the negative/print.
Framing is just moving a "frame" around in a picture. For example, make a 4x4 frame out of toothpicks or something like that. When you place that on a standard piece of paper, the area in the middle is "framed". And you can move that frame to different parts of the page, and, in essence get different pictures. You are not changing the proportions of the picture, just the area that you are showing/developing/printing.
Some of the kiosks allow you to frame different parts of a picture - like just a face or one particular person in a photo.
I'd say the easiest thing to do is just to upload some pictures and start fooling around with the editing options. Then you can "preview" your pics on your computer screen. No, they won't be on paper, but if you did things like change the contrast, added some sort of effect, or even resized the pic, then you can see what it will look like once it's on paper.
Now if the pics you were given were scanned in and not directly from a digital camera, then that's a different issue. You may just end up with some prints with black bars at the sides and/or top and bottom. -That happens with panaromic prints that are printed in 4x6 rather than 4x10 (or whatever size).
And if someone says there camera takes prints in different sizes - 3-1/2x5, 4x6, etc. - then they're "lying" to you in sense. There may be optimum enlargement sizes due to the number of pixels, but the "negative" is pretty standard regardless of the final size of the photo.
Hope this makes sense.
