What program are you using to resize it?
Programs like Paint and Photoshop have an edit or image menu where you can change the dimensions of a picture in pixels. You want to get it to 100x100 pixels. Save it and find it from the CP on tech.
EDIT: Here, just copy it and save it to your computer and upload it to ls1tech.com
Also, I noticed you posted this in the testing area. Next time, post it in the support and resources are so you can get more people to view the thread.