To make your photos smaller, you need photo editing software. There are lots of options. ACDSee
(Classic, i.e. the plain jane version) comes bundled with the HP Photosmart cameras, and it's a good piece of work -- very user friendly. It's also readily available for not a lot of $$, and the latest version, which has more bells and whistles, is not a great deal more expensive.
However, there are lots of others, too. Whichever you settle on, you need to find a feature called "resize" or something to that effect, and make the largest side of the image no more than 400 pixels. That will give you an image the site will accept for most situations. You should make those images even smaller, though, if you want to put them in your signature, say 200 pixels on the largest side.
And to put them in your signature, you also need to find a site to host them, which allows remote linking. I use either Village Photos
which are not expensive. Some members have also found free hosting services, though I wonder how long they'll stay that way: Village Photos was free when I started there, but they changed their account structure so that the free account does not allow remote linking anymore.
Anyway, there's some info to get you started. Others will undoubtedly reply with their favourite software and hosting sites. Details about how to actually get that edited picture into your post are in this thread
And we'll look forward to seeing your pictures. Good luck!