You have to have them as zip files dunc, cos they won't upload unless they are zipped. If you look at the attachment panel when you're typing a message you see an underlined blue 'allowed extensions and sizes' link. Click on it and it will tell you all the things you can upload.
You'll notice that even though you can upload jpgs as single files, if you have several in a folder you have to zip it.
When I zip a texture it is only slightly less in size than the unzipped folder, probably because it contains jpgs that have already been compressed.
If you're finding that the zipped file is bigger than the unzipped one then there is something strange going on at your end. With models the zipped folder can be as much as half the size of the unzipped one.
