I intertwine between SLumped and XWE. They both have their own bugs and features, so really, it's a matter of what the task is. If you're editing really large files, I'd recommend SLumped, since XWE autosaves (which is a bad idea by itself), but XWE works better for things such as character wads.
As for the topic at hand, set the IWAD for your wad editor to SRB2.SRB. This should fix your graphics problem.