Why won't images upload to my site?

[Coleslaw]

Hi
My site has been working perfectly for months. Am trying to add some new products to my website (something I do regularly) BUT my JPEGs will not upload in the admin panel when I am typing all the details for the new product.
It is weird because even if I try to use images that I have used elsewhere already, they still won't upload (so it isn't a problem with my new product pics - JPEGs). All the product images that are there already are fine. I just can't add new products because the image will not upload.
Can anyone help me? I have some new stock to add and this is so frustrating. I have asked the person who built my site and she is perplexed too and looking into it.
Many thanks

[michael_s]

Two things to check:

1. Make sure you are not over your hosting account disk quota. Images will not upload if you are.

2. Make sure your images directory is world writable - chmod 777