GiFFY is a free utility that can convert .bmp files to .gif format and vice-versa. With just a few clicks, GiFFY can create transparent, non-transparent, interlaced, and non-interlaced GIFs. One reason you'd use GiFFY is that it can fix .gif files which some Internet browsers (particularly some versions of Netscape) can't read in Java applets.

To create menu associations with .gif and .bmp files run Setup.bat. After you do so, you may just Right-Click any .bmp or .gif file and choose what you want to do with it.
(Hint: you may select several files using Ctrl key on your keyboard.)

if you choose GiFFY. Convert to Transparent GIF color of bottom-leftmost pixel of the bitmap image will be used as transparent color. To set your own transparent color choose Open with GiFFY and save your GIF manually.

GiFFY supports both Graphical and Command line interface.
Using Command line:

GiFFY [ /a | /t | /ay | /ty ] source [target]

Convert switches:

/a - normal mode
/t - if target file is .gif it is saved with default transparent color.

/ay and /ty - are the same switches, but when you use them GiFFY will not ask for your approval to overwrite the target file.

Examples:

GiFFY mypic.gif - starts GiFFY with mypic.gif loaded.

GiFFY /a mypic.bmp - converts mypic.bmp to mypic.gif

GiFFY /t mypic.bmp - converts mypic.bmp to mypic.gif using default transparent color.

GiFFY /a mypic.bmp smallpic.gif - converts mypic.bmp to smallpic.gif

GiFFY /a bad.gif good.gif - converts bad.gif to good.gif

GiFFY determines GIF/BMP type by file's extension.
When using long filenames with spaces you should quote the file name, like: my picture.gif

How Conversion Works:
GiFFY does not make any changes to the original files.
If the target file name exists dialog box will appear asking you to rewrite the file.

This software is based, in part, on the work of Anders Melander.