Make sure your images have extensions. They should not just be named "tSSbackground", but instead "tSSbackground.jpeg" (or whatever extension they are).
I can't answer regarding specific sizes since I'm really not a graphics person. Perhaps Shelley or someone else who is good with graphics can answer that question. Regarding photoshop, make sure when you save the images, you Save for Web. That will make the images smaller in size for the web.
As for repeated, for instance the background on this page looks like this and is repeated vertically (I'll link to it since it is so long) -
https://vborg.vbsupport.ru/images/cs...ybkgd950px.jpg