Can't upload logo…

I’m trying to upload a logo and I have a number of problems:

– No file formats are being accepted. What is allow to be accepted?
– Google chrome and other browsers crash when I try to upload to elect an image.
– The one image I did get uploaded shows up in the header but it is extremely small. Why?
– How do I format logos to be the required sizes? I am working on a MAC.

If anyone knows how to handle any of this I would greatly appreciate the help.

Thank you,

Shane