I'm not sure it should have "animator" in the title as it doesn't actually animate anything :PYes. You are absolutely right. I have invented few names for this software.
You may want to consider how the image will end up if there are many frames: does it only stick to a single column/row?
The amount of wasted space when mixing differently sized images can be rather...wasteful. You may want to consider if keeping them in order is more important than re-arranging them and packing them more tightly as orders can be re-assembled after loading. You could even save a text file (or xml/json) with the rectangles of each packed image ;)
Note that you can do the same thing (and more) with Imagemagick : http://www.imagemagick.org/Usage/montage/#montage