As cleanspade said... Use Paint. Heres how.
1. Right click on the image you want to use, a menu will pop up. Click on "Open with" Choose paint. A new window will open with Paint and your Image will be visible, even if it is only the corner, its because your image is so big - dont panic.
2. Up in the top left of the window you will see an icon for "Resize" Click this and a small window will pop up. You will see the percentages is clicked, instead, click on pixels next to it, then change the biggest number in the top box e.g. 4752 to around 750, then click ok. Ta da! your image should now be small enough to upload.
3. Just one more bit, you have to save your handy work. Again, up in the very top left, under the small palet and brush, theres an icon that looks like a page with text on it with a small arrow, click this and you will get a menu, you looking for "Save as" click this and choose "jpeg" You now have to change the new file name - call it what you like to make it easy to find but I normally just add THL to the end. I should be saved in the same folder you took the original image from.