on the upload page
HERE, there is a link to irfanview, which is a very handy tool, like paint but much better which lets you resize images and save them.
To resize, open your image and press Ctrl+r, type in width as 800 px and press OK.
To save under 100Kbs hit the save as button, and adjust the quality slider to somewhere between 1 and 12. You might have to play around with this a bit to get it under 100Kbs. If it is not, just save it again at a lower quality setting

Any questions, just ask

.mic
PS. If you have photoshop, after resizing hit the "save for web" button in the file menu, then you can play around with quality from 1-100 and it gives you a preview of the file size so there is less playing around
