Mikah (and Dave)
Yes it is possible to do it in Photoshop. There is more than one way but if you are familiar with 'copy' and 'paste' then this is as good a way as any. Basically, once you've got your photo ready, have it open in 'Photoshop' then go to: File>New and in the box that opens select A4 size (say) by making the dimensions 21cm x 29.7cm and select the same resolution as your photo - this will give a file size of about 25Mb if you have chosen, say, 300ppi resolution. Choose 'transparent' background colour and the checkerboard 'pattern' which comes up (which won't appear in the print) will help you position your shot accurately - this is really useful if you want to put several shots (which will need to be the same resolution) on the same page.
Now 'click' on your photo, and press Ctrl+A which will 'select' it followed by Ctrl+C which will 'copy' it. Now click on your new blank A4 image and press Ctrl+V which will 'paste' it onto the middle of the area. Use the 'move' tool, which in Photoshop looks a bit like this: >+ (near the top right of the tool pallette) to move it where you want it on the blank page. If you're putting more than one image on you can move them around via the layers box, which if it is not open already, is accessible via Windows>Layers you then just click on each layer as you wish to move it around on the blank background.
When your image(s) are in the final position hit Ctrl+E to 'merge' the layers into one image then Ctrl+S to save. Don't forget to select A4 size paper in the printer software program (Epson printers, for instance, annoyingly default to 8 1/2x11 1/2 paper size!) to make sure it prints exactly where you want it. Also, with Epson, if you've positioned it close to the edge make sure you select 'maximum' area.
Other image-editing programs should have similar functions to the above.
OK - it looks complicated but once you've tried it you'll realise how easy it is - honest! I'm very much a computer-duffer and if I can do it anyone can. If you've got a fast computer it really is only a matter of a few seconds, the slowest parts being making the new blank area together with the merge and save if you've chosen a high resolution like 300ppi or more. The actual 'select, copy, paste, move' bits are very quick and easy.
Adey
