Not sure how you did that--FFF doesn't do conversion to gif. Must be something in convert.
There is, however, a setting in FFF for resizing larger images:
Code:
## Resize images down to width, height, preserving aspect ratio.
## Nook size, with margin.
image_max_size: 580, 725
FYI, Calibre's convert can also resize images depending on Page Setup -> Output Profile. I don't the details except that 'Tablet' doesn't resize.