If the pix are gif or bmp, convert them to jpg. bmp files are way too big for the Internet and gif files are only good for pictures with only a few colours (line drawings, BW comics etc) or for animated pictures (you cannot animate a jpg).
If they are already jpg, or if they are still too big after you have converted them, you can reduce the 'quality'. Anything above 60% is OK for thumbs, and you can usually go as low as 50% (but only on thumbs, not on your big pictures).
Next I assume you are going to ask how you do all this. Well any good graphics package will have these functions. But I am guessing that as you are just starting out you do not have several hundred dollars to splash out on one of those. So take a look at at IrfanView, I think it is still free. It is a picture viewer, but it does have a few simple picture editing functions such changing image size, cropping images, adding text, and doing the conversions I mentioned above. For many of these things it is as good as a package you would pay several hundred dollars for. In fact for batch conversions of jpgs or for size reductions, I prefer it to the big graphic package I have, it is quicker and simpler to use.
But for you, the best part is it is free (or at least it was, I have not checked since the last time I had to download it when I killed my hard drive).
It is at:
http://www.irfanview.com/