1) Get yourself some image editing software - there are many available, some expensive, some not. If you want a free one, I recommend the GIMP, which also has a windows port, available Here
2) Open up your image in said software
3) Resize your image to something around 80*80 pixels, preserving the aspect ratio of height to width.
4) Save as a .jpg so that the size of the new file is as small as you can easily make it.
5) Hot link to tMP (preferred) or upload to tMP.
1a) Send the image to Kepps, and he'll do it for you (neal at themouthpiece dot co dot uk)