Export drawings in Altium Designer
First step is to generate a smart PDF from your project, File \$ \rightarrow \$ Smart PDF. The generated file will contain all of your drawings starting from schematics to PCB layouts, in vector graphical format.

The next step is to dowload a software like this, which is a PDF to GIF converter.

This software extracts all images from the PDF and one by one converts them to gifs.

Here is the result from one of my project, all files converted:



The quality is satisfying, this one is a [maximum zoom in] in a Firefox browser:



This method ensures better quality and faster then making and saving single screenshots from every sheet.