I'm making a simple website with a top nav frame and the lower main frame. I'd like a curve in the top left and top right corner of the main frame but for this to remain visible when the content is scrolled up. I've made a gif for the left and right corner. How would I achieve this?

You mean like freezing the header so that when you scroll up or down it remains in the same place??

Yes, but the header on the lower frame, so that it looks as if the image in the top frame curves round into the lower frame. The curves (gifs) should stay in the top left and right corner when the user scrolls.

2 options.
Option 1 : Created another set of frames sub_header and sub_lower.
Option 2 : Still with frames but this time just create an iframe and put all your content in the iframe.

Its cheap but thats how i did it. lol.

Use gifs in the top frame instead of the main one. You'll need to invert them but they'll stay fixed in the top frame