Resizing, sometimes called resampling, requires an external program on your computer. You can use MS Paint which should already be there. Other options include Picasa http://picasa.google.com/index.html which is free, Paint Shop Pro (free tiral then you have to buy), Photoshop (FAAAR from free) and a ton of other options.
I haven't tried a large number of the free tools available out there (Picasa and GiMP aside) hopefully others will chime in here - I'd be interested to see whats out there and opinions on them.
You can also upload your photos to your gallery available on site here which will then give you preformatted "tags" for the pictures (it will pop the right IMG tags in there).