FYI since I know you use FastStone, you can also resize any image that you like that is on your computer with FastStone.
To resize or check the current size of an image in FastStone, open Fastone and click on the image. Go to Edit/Resize. A window will open where you can view the current size or enter new dimensions for upload.
You can also find a new image online that you like, save the image your computer and then resize it as above.
Tip on searching images in Google: You can tell Google which size file to search under Google Advanced Search. Scroll down to image size and use the drop down to specify.
When you go to upload an image for your profile here, you will also have the opportunity to do some adjusting and cropping in case the image is still too big. So you might even try experimenting right now with uploading a picture and try to upload something that is already on your computer, play around with the crop a bit and see if it will take the upload.
As slowlane stated above, the system won't allow it to upload if it is too big and it will rescale it.