bit of photoshop help needed before printing

[tim]

I had a few photos printed today, and I sent instructions that each picture had to be within the size 24*19cm, so they'd fit in my frame. I said not to crop or stretch them. Well, they couldn't do it, so they just printed them on standard 8*10 inch sizes.

What they told me is I have to resize them in photoshop and add empty borders, so that when they're printed 8*10 the pictures come out being the size I want.

Now I have a few ideas how to do this, basically changing the canvas size, but i'm having a bit of a hard time working out exactly what figures to use. It doesn't help that two of the photos are 3*2 ratio and one's 5*4.

If anyone knows of a guide anywhere that could help me out, i'd appreciate a link. Or if it's simple and someone can be bothered writing down a few tips, that'd be great too :)

[tim]

Ok, I figured it out myself, it's really simple in photoshop:
1) Go to Image | Image Size, turn off resample image, and enter the length of your longest size. eg I want the image to be 25cm long in portrait mode, so I entered 25 in the width box. You will probably have to use the height box for portrait pics.
2) Choose Image | Canvas Size, turn off the relative check box, then enter the width and height of the paper it'll be printed on. For example, my photos will be printed on 10*8 inch paper, so I entered that.
3) Save and send!

Strangely the lab I went to only (one with a pretty good reputation) accepts only JPGs, not TIFFs - is that normal?

Edit2 - i'm pretty sure this is correct.