A CHM file is a self-contained file system, equivalent to a hard driver. You cannot refer to items "above" the root of a CHM file. It is similar to that you can not refer to items "above" the root of a hard driver. Unlike real hard drivers, you also can't access other "drivers" through an inter-CHM file jump because a CHM file only has one "driver".
Let's say you've got the following directory structure:
and in default.htm you have an IMG tag:
Since \projects\test is where your project file is, that is also where the root of your CHM file is, which means you have tried to refer to a file in a hierarchy above the root -- which is simply not possible. That's like saying if my current directory is c:\ then I want to refer to a folder whose hierarchy is higher than it -- you can't because there are no folders higher than the root. What is supposed to happen at this point is that the compiler recognizes that you have referred to a file above the root of the CHM file.