Assuming that you've taken the pictures with a digital camera it probably came with photo manipulation software which will do the job for you. Indeed, any photo manipulation software can do it. (Simply look for a 'resize' or 'resample' option).
If you've not already got some suitable software (or you've not got a clue how to use it) download and install the excellent (and free!) Irfanview from here:
http://www.irfanview.com/
At it's most basic level it's a really good image-viewing program, worthy of a place on everyone's computer. (It's vastly better than the image-viewing software which comes with Windows). But it's also got lots of options for manipulating images (including resizing them) built into it.
Instructions for resizing images:
Open Irfanview and then open an image file (so that you can view it) in the same way that, for example, you'd open a file in Word. i.e. go to File>Open (or simply click on the 'open folder' icon underneath 'File'). Navigate to the required file and double-click on it.
Then go to Image>Resize/Resample.
Now choose the required image size. (There are loads of options here but I suggest simply using the drop-down menu to select '640 x 480'). Click 'OK'
That only changes the size of the displayed image. i.e. it hasn't changed the size of the file on your hard drive. To do that you need to go to 'File> Save As'. (NB: DON'T USE 'File>Save' AS THAT WILL OVER-WRITE THE ORIGINAL FULL-SIZE FILE!!!)
Enter a REVISED name for your file, so that you can tell the big and small files apart. (e.g. if the original file is called 'birthday', use a name such as 'birthdayx' or 'birthdaysmall' for the smaller one which you're about to save.
Use the drop down menu to change the 'Save as type' field from 'BMP' to 'JPG'.
Click 'Save.
Then email 'birthdayx' (or whatever) instead of 'birthday'.
Chris