What means "does not work"? Please specify more precisely.
You may try the following:
- make sure that with each trial that you don't load the page from cache (this kind of problem drove me crazy in the past)
- leave away the ./ or ../, simply use the relative path (e.g. admin/images/yello.jpg)
- try to load the jpeg file by entering the path in your browser (if this fails, fix the path and use it in your CSS)
- if the above all fails: give more detail so that we can give more prcise advise ;-)
Cheers
Andi