If what you're talking about is the artwork for the Channel Store and the Home Screen then the SDK contains a document, "Homescreen and Channel Store Artwork Guidelines", in the file artwork-guidelines-v3.pdf. This document gives Roku's recommendations for this artwork, including different sizes for SD and HD images. So yes, you can make different images for SD and HD, which seems to be what they recommend, although in practice it seems that you can use the same image for both SD and HD, and this is what several of the examples in the SDK do.