It sounds like something you would do on your computer before uploading, not online through wikitree.
If you still have the image on your computer, open it with an image editing program that you are comfortable with... any one will do. Change the size of the image (in some programs that choice is listed in the properties catagory). Save the new image with a new name, then upload it and you should have no trouble. An image can be made as large as 8.5 x 11 inches at 300 dpi for example, or bigger. I think 10 meg is the largest accepted.
Good luck. I hope this helps!