Generally speaking, most designers make "master documents" using all the features of Photoshop, including Paths, Vector Layer Masks, etc., and save them as .psd files. Then they do Save As (or Save For Web & Devices) to create work products, such as the final images for publication on the web.
In your case, you might choose to make your design using fill layers masked by vector masks, which are made by starting with Paths. The edges of these will generate anti-aliased transitions between colors, can be easily edited (e.g., to change color) and can be scaled well.
I don't know what else is in your design, but if it's just block and line art some might propose using Adobe Illustrator for the task, but Photoshop is quite capable if that's what you use and are familiar with.
Consider this very simple design:
Make sure that you are working in RGB (not indexed color) mode.
If you are working with GIF then I would imagine that the image should not be magnified. GIF is a file format that is typically displayed as a screen graphic. There is no point in working/viewing at anything other than 100%.
Many thanks to you both (I have saved your answers).
Yes, the diagonal lines/columns in the Water System graphic are exactly what I mean.
I haven't got Illustrator, so I will need to try it out with Photoshop. Beforehand I had better read up on vector masks and paths!
Thanks again to you both.