When you download ImageGrab from my site, which is the best way to have the latest version and be sure that it has nothing malicious, you get a compressed file like imagegrab_xyfr.zip that contains a readme.txt and an executable named like imagegrab_xyfr_setup.exe. You understand that xy represents the version, for example 61 for version 6.1.
It is of course the setup file that must be launched after unzipping, and then just follow the suggested directions. After installation the application can be started directly.
ImageGrab installs in a very respectful way of your system, without disturbing at all what is already installed, and in a perfectly reversible way. It has the particularity of using a version of the famous filters directshow (open source) LAV and ffdshow, but by performing a “private” installation that does not interfere with the LAV or ffdshow filters, or any other application related to video , which would already be installed on your system.
A portable version should be available soon.