1. Get gimp
2. make a new image the size of your frames
3. import images in sequence (if you have alot you can do them all at once by putting them in order in a folder, selecting them all and dragging the first image into gimp
4. save as a gif image, selecting "save as animation" from the options
5. select replace as the default mode for frame disposal in the save dialog
6. ???
7. GIF image
to set the time different for individual frames in a gif add (xms) to the end of a layer's name in milliseconds
(To get to the layer dialog, you have to open it with ctrl + L or go to Dialogs>layers)
for example: (100ms) for 1/10 of a second or (500ms) for 1/2 of a second
