software i'm looking for

i am on the hunt for a command line program that takes an image file in one of the major image formats (at least gif, jpg, and png) and draws a straight line in it, vertical or horizontal, at the specified position, in the specified color. or as an alternative, one that draws a rectangular box at a specified position and size in a specified color. i need to do this as part of an automated script so i need to find one that is totally non-interactive.

Imagemagick will do all that loads of examples on the web, it should be in your distros repos

ImageMagick's convert utility can add basic lines and rectangles to images using its draw option.

I've played with jpg and png files a little.

Too late...

There's a user guide available: ImageMagic