Why does mencoder output still images if I try to encode them under 3 FPS, while mencoder outputs dynamic images if I output them at 3 FPS or over?

If I try... mencoder mf://@NetSWLW12MA.txt -mf w=800:h=600:fps=1.5:type=png -ovc lavc -lavcopts vcodec=mpeg4:mbd=2:trell -oac copy -o NetSWLW12MA2.avi I just get a bunch of still images of the first frame. if I try... mencoder mf://@NetSWLW12MA.txt -mf w=800:h=600:fps=4:type=png -ovc lavc -lavcopts vcodec=mpeg4:mbd=2:trell -oac copy -o NetSWLW12MA2.avi then I get mencoder to robustly play all the images. (there are 145 separate image frames contained in the txt file). Is there a way to put all 145 images as separate images in the mencoder file when I encode it at 1 FPS?