Another 'resolution' question

Hi,

I have read a couple of other related threads, but I need a little more help to resolve my 'resolution' confusion...

I am applying to a stock agency, and see that one of their requirements is

"...If necessary, interpolate (upsize) the file to 24MB using a specialist, professional software package. If you have a camera that is capable of producing an uncompressed 8 bit file size of over 24MB then leave it that size. We recommend Adobe Photoshop although other software is acceptable. If using Photoshop version 7 or higher select the bicubic option. We advise that you do not use “step” or incremental interpolation. Check your software’s default settings to ensure that all sharpening is turned off..."

My 10 mp camera typically produces high quality versions (jpegs - Aperture 3) of about 8-10mb. I need a nudge in understanding what 'upsizing' and 'interpolation' is - and even more important for me - can this be done in A3 or do I need to buy and learn PS? (I've used PSE in the past, but for two years pretty much only use Aperture except in rare situations.

Thanks very much,

Greg H.