Unless you're planning on enlarging pictures to greater than A4 size, you really ought to change the settings on your camera anyway. A 10Mp camera doesn't produce any better pictures than a 3Mp camera at sizes up to A4. All that you're doing at the moment is taking photos which take longer for your camera to process (so you have to wait longer before you can click the shutter again), take up more space than necessary on your media card (so you can't get as many pictures onto a card), take up unnecessary space on your hard drive, take forever to be emailed, might be rejected by some email servers and won't be accepted by sites like eBay.
A picture for use on a site like eBay ideally needs to have a file size of around 50Kb. (I'd regard 30Kb as a minimum and 100Kb as an absolute maximum). Your old camera was probably producing pictures with a file size of around 500Kb (i.e. 10 times as big as required). Your new camera is probably producing pictures with file sizes of around 2000Kb (i.e. 40 times as big as required).
Reducing file sizes is easy but the exact method depends upon the software you're using. Both of your cameras almost certainly came with image manipulation software to do the job. (Just look for a 'resize' option). Otherwise Irfanview is a superb free program (for both image viewing and image manipulation) which deserves a place on everyone's PC:
http://www.irfanview.com/
Open your image in Irfanview. Go to Image>Resize/Resample
You'll see loads of options. For your basic needs, simply click on '640 x 480 pixels' and click 'OK'.
Then go to File > Save As and save your file with a new name. (e.g. If your original picture was shoes.jpg, call the new image something like shoesresize.jpg).
[NB: Always use 'Save As', NOT 'Save', otherwise you'll overwrite the original image file].
Chris